WASHINGTON — As federal workers file out of the State Department at the end of a Washington workday, an elite group is often just arriving in the marbled, flag-lined lobby: Billionaire CEOs, Supreme Court justices, political heavyweights and ambassadors arrive in evening attire as they're escorted by private elevator to dinner with Secretary of State Mike Pompeo.

Until the coronavirus shut them down in March, the gatherings were known as "Madison Dinners" — elaborate, unpublicized affairs that Pompeo and his wife, Susan Pompeo, began in 2018 and held regularly in the historic Diplomatic Reception Rooms on the government's dime.

An NBC News investigation found that Pompeo held about two dozen Madison Dinners since he took over in 2018. NBC News obtained a master guest list for every dinner through the end of 2019, as well as internal State Department calendars from before the pandemic emerged, showing that future dinners were on the books through at least October. The master list includes the names of nearly 500 invitees and specifies who accepted, although it is possible some people RSVP'd but didn't show up in Foggy Bottom for dinner.

The records show that about 29 percent of the invitees came from the corporate world, while about a quarter of them hailed from the media or entertainment industries, with conservative media members heavily represented. About 30 percent work in politics or government, and just 14 percent were diplomats or foreign officials. Every single member of the House or the Senate who has been invited is a Republican.

The dinners are named after James Madison, America's fourth president and fifth secretary of state, who made a habit of inviting foreign diplomats to exchange ideas over dinner. But historians could point to no precedent for a secretary of state's playing host to such frequent gatherings, paid for by State Department funds, involving political and business leaders.

"Madison certainly paid his own entertainment expenses," said Kevin Gutzman, a professor at Western Connecticut State University who wrote a biography of Madison.

The Madison Dinners, which aren't disclosed on Pompeo's public schedule, add another element to what his critics say is a pattern of pushing the edge of the envelope by using government resources for potential personal or political gain.

Steve Linick, who was abruptly fired Friday evening as the State Department's inspector general, was investigating whether Pompeo made a political appointee carry out personal errands like walking his dog, NBC News reported.

On Tuesday, a State Department official and two other people familiar with the matter identified the political appointee to NBC News as Toni Porter, who had also worked for Pompeo at the CIA and now works in the Office of the Secretary of State. Emails reviewed by NBC News show that Porter was the chief liaison between Pompeo's office and the Office of the Chief of Protocol, which runs the Madison Dinners.

It's unclear whether the inspector general was also investigating the Madison Dinners, but two administration officials told NBC News that Linick made some type of inquiry to the protocol office last week, before he was fired. One of the officials said Pompeo's office was then notified.

When the dinners started, two State Department officials said, concerns were raised to the State Department's legal adviser, who they said responded by saying events hosted by the secretary should be related to foreign policy. On Capitol Hill, several committees have also been looking into the dinners, congressional aides said.

In the opinion of a senior Trump administration official who requested anonymity out of concern for retribution, "if the president knew about any of this, he would have fired Pompeo months ago."

In a letter Tuesday to the State Department reviewed by NBC News, Sen. Bob Menendez of New Jersey, the top Democrat on the Foreign Relations Committee, wrote: "I am concerned by allegations that the Secretary appears to be using those taxpayer resources to host large domestic-focused political gatherings that serve little-to-no foreign policy purpose." The letter requested "a complete accounting" of funds used for the dinners and copies of any remarks Pompeo delivered at a Madison Dinner.

State Department spokeswoman Morgan Ortagus said the dinners are "a world-class opportunity to discuss the mission of the State Department and the complex foreign policy matters facing our exceptional nation."

"Invited guests have included many foreign diplomats, thought leaders, academics, government leaders at many levels, business leaders, Members of Congress and the media — each of whom has a stake in America and its leadership in the world," Ortagus said.

She added that Pompeo "has benefited greatly from these gatherings as he has gained knowledge listening to his guests from all across the political spectrum and all around the world."

"Foreign policy-focused social gatherings precisely like these are in the finest tradition of diplomatic and American hospitality and grace," Ortagus said. "The Secretary looks forward to continuing these Madison Dinners as they are an important component of the execution of his duties as Secretary of State."

An invitation to an 'intimate evening'

Invitations obtained by NBC News describe the dinners as an "intimate evening" in the spirit of James Madison, who as secretary of state "hosted dinners that gathered thinkers and leaders to share ideas on the future of America and the World."

"Through the Madison Dinner Series, Secretary Pompeo honors their wisdom in seeing the value of building relationships and sharing intellectual thought to enrich our country and to further our diplomatic goals," an official invitation reads.

An invitation to a Madison Dinner from Secretary of State Mike Pompeo, obtained by NBC News.via NBC News

Many of those invited are, indeed, global thought leaders whose perspectives could be valuable to America's top diplomat: foreign ministers from allied countries, senators and prominent historians. But others seem to have little connection to the world of diplomacy, such as country singer Reba McEntire, New Hampshire Gov. Chris Sununu and NASCAR driver Dale Earnhardt Jr.

At one of the final dinners before the pandemic, as President Donald Trump was edging away from a potential military confrontation with Iran in January, Pompeo and his wife hosted the Saudi ambassador to the U.S., Princess Reema bint Bandar; Raytheon CEO Thomas Kennedy; national security adviser Robert O'Brien; and Fox News host Brian Kilmeade, according to a seating list reviewed by NBC News.

At a dinner in November, as impeachment hearings were in full swing, the Pompeos dined with former Major League Baseball Commissioner Peter Ueberroth, American Gaming Association President Bill Miller and Marjorie Dannenfelser, president of the Susan B. Anthony List, the influential anti-abortion rights lobbying organization. The Malaysian ambassador and Rep. Lee Zeldin, R-N.Y., a key defender of Trump during the impeachment proceedings, rounded out the list.

Then there was the dinner in May 2019 that occurred after Trump stormed out of an infrastructure meeting with House Speaker Nancy Pelosi, D-Calif., who accused him of throwing a "temper tantrum." Among those invited for the next evening's dinner were Fox News host Laura Ingraham, the EU ambassador, Republican power couple Matt and Mercedes Schlapp and Chik-fil-A Chairman Dan Cathy, a major donor to campaigns against same-sex marriage.

"The CEO of Chick-fil-A is not someone I would say is involved in foreign policy," a person with knowledge of the dinner said.

NBC News obtained a master guest database that details each guest's name, title, spouse, dietary restrictions and dates of invitation and acceptance. The database runs from July 2018, a few months after Pompeo was sworn in, through the start of 2020.

An invitation to a Madison Dinner from Secretary of State Mike Pompeo, obtained by NBC News.via NBC News

The business moguls invited to the affairs include AOL co-founder Steve Case and Republican megadonors like Home Depot founder Ken Langone, hedge fund executive Paul Singer and Texas real estate tycoon Harlan Crow. The invitees also include Pompeo's old classmates from the U.S. Military Academy, including Joe DePinto, CEO of 7-Eleven, and Steve Cannon, whose company owns the Atlanta Falcons.

Twenty-three percent of invitees have been associated with media or entertainment, and another 30 percent are government types: current or former officials, members of Congress and judges and their spouses. Media figures skew heavily toward conservative TV personalities, with 39 percent of them from Fox News.

Only 14 percent are diplomats or foreign officials. More than 50 ambassadors show up on the guest list, with nearly two-thirds from countries in Europe and the Middle East and smaller numbers from Asia, Latin America and Africa.

Many are major players in Republican politics, such as former Bush strategist Karl Rove and David Urban, a lobbyist and political consultant on Trump's 2020 advisory committee. Supreme Court Justices Samuel Alito and Neil Gorsuch appear on the list, as do Treasury Secretary Steven Mnuchin, Commerce Secretary Wilbur Ross and Defense Secretary Mark Esper, although it's unclear whether all of them attended.

The master guest list includes many notations on specific invitations being handled directly by Susan Pompeo, including those for Sununu, the New Hampshire governor; Dr. Larry Beamer, a surgeon in Wichita, Kansas; and historian Niall Ferguson. Pompeo, a former House member from Kansas who entered political life without significant wealth, has presented himself publicly as more of a low-key politician, especially compared to his last two predecessors at the State Department, who were extremely wealthy.

"I'm not in it for the fancy dinners in Paris or Switzerland or Vienna," Pompeo told the American Conservative Union Foundation in February, alluding to frequent European trips by former Secretary of State John Kerry. "That, my friends, is a lot of cocktails."

The night before those remarks, NBC News journalists had spotted guests arriving in the State Department lobby for pre-dinner cocktails as one of the last pre-coronavirus gatherings got under way.

Cocktails and conversation

The evening typically starts at 6 p.m. Guests arriving by car pull in to the horseshoe driveway of the Harry S. Truman Building, the headquarters of U.S. diplomacy, where officials are waiting to greet and escort them to a special elevator reserved for the evening, people present for the dinners said.

Once upstairs on the eighth floor, they're given a walking tour of the iconic Diplomatic Reception Rooms, a museum of U.S. diplomacy that includes the Benjamin Franklin State Dining Room, the Martha Washington Ladies' Lounge and the John Quincy Adams State Drawing Room.

Then it's cocktail time for roughly half an hour before the guests are seated for dinner, which lasts about an hour. The secretary or his wife tap their glass and give a welcome toast, but the conversation is informal, and there's no specific topic or theme.

The menu, however, is often curated to reflect the season. The featured cocktail might be one paying homage to the time of James and Dolley Madison, or French 75 cocktails might be served up in honor of Mardi Gras. At a dinner in February 2019 occurring the same week as Mardi Gras, guests were served king cake, a New Orleans tradition with a baby figurine baked in.

"It's not the secretary of state of the United States. It's more like Secretary Mike and Susan," said a person who's been at one of the dinners.

A checklist for the events shows that a harpist is brought in to play the cocktail hour, and a photographer is arranged to take a group photo in front of a fireplace.

As guests depart, usually around 9 p.m., they're given a journal and a pen as gifts — both custom-embossed with the Madison Dinner logos. The State Department special-ordered hundreds of each in 2018, the pens for $23.75 apiece and the journals for $8, officials said.

Pushing the envelope on precedent?

Madison was, indeed, known to hold salon dinners in the early 1800s to build bridges on foreign policy between Americans and foreigners. Hospitality, public diplomacy and the informal exchange of ideas are among the traditional tools secretaries of state use to promote democratic values and advance U.S. interests, and other secretaries of state have held social events to expand their networks and conduct informal diplomacy.

"The dinners are not unusual," said former Republican National Committee Chairman Michael Steele, a Trump critic and MSNBC contributor who attended one of the dinners. He said the dinner was in keeping with Pompeo's diplomatic role and added: "There's no big deal there."

But the manner in which Pompeo has carried out the dinner series has raised concerns for State Department officials on multiple fronts, including the use of taxpayer dollars and the involvement of his wife.

Former officials said that it's up to each secretary to decide how involved their spouses should be and that no regulation prohibits a spouse from playing the role of de facto social secretary. Susan Pompeo, who isn't a government official, was also known to play an active role when Pompeo was CIA director, and although he has called her a "force multiplier," her presence on official State Department trips has raised questions before.

In the Madison Dinners, Susan Pompeo's role is central. Emails obtained by NBC News show her communicating directly with State Department officials, setting guest lists and dates to be put on the calendar for dinners, and arranging menu choices.

Officials involved in carrying out the dinners say all the information collected by the State Department during the invitation process, including the names and contact information for potential guests, is emailed back and forth to Susan Pompeo's private Gmail account. Two congressional officials expressed concern that information could then be used by Pompeo as a potential donor Rolodex if he runs for office again. The Hatch Act prohibits most federal employees from engaging in political activity at work or in their official capacities.

Pompeo has resisted ongoing efforts by Senate Republicans to recruit him to run for the Senate in Kansas this year, and he is also often mentioned as a potential 2024 presidential candidate.

The dinners are paid for out of the State Department's Emergencies in the Diplomatic and Consular Service Appropriation, known as the "K Fund," which can be used for "confidential requirements in the conduct of foreign affairs as well as other authorized activities that further the realization of U.S. foreign policy objectives," according to the State Department's website. Current and former officials said that to comply with the spirit of the appropriation, State Department officials generally work to ensure that a significant proportion of foreign dignitaries or officials attend.

Unlike for most State Department events, cost estimates for the dinners aren't circulated to staffers working on the events, officials said. But two sources with knowledge of the events and the underlying costs estimated that the bill probably runs several hundred dollars per plate, likely pushing the total cost of the dinners to date into the six figures.

The dinners also circumvent the normal procedure for an official event for the secretary, in which an "activity sheet" is sent out from the secretary's office notifying other officials of the plans. People familiar with the Madison Dinners said they're instead arranged informally between the Office of the Chief of Protocol and Pompeo's executive office on the seventh floor.

That means there's no coordination with the desks at the State Department overseeing specific regions of the world. For example, if the South Korean ambassador goes to a Madison Dinner, the State Department's bureau handling Asia may not even be aware or know to follow up to see what the ambassador and Pompeo may have discussed, officials familiar with the dinners said.