The FBI is investigating the alleged harassment of a Joe Biden campaign bus last week by motorists displaying Trump 2020 flags, an FBI spokesperson confirmed Sunday.

"FBI San Antonio is aware of the incident and investigating," FBI spokesperson Michelle Lee told CNN.

The incident took place in Texas on Friday as the campaign bus was traveling from San Antonio to Austin as part of a push to urge Biden supporters to cast their ballots on the state's last day of early voting. A Biden campaign official described the motorists' actions as an attempt to slow down the bus and run it off the road.

People in vehicles that were part of a "Trump Train" began yelling profanities and obscenities and then blockaded the entire Biden entourage, according to a source familiar with the incident.

At one point they slowed the tour bus to roughly 20 mph on Interstate 35, the campaign official said. The vehicles slowed down to try to stop the bus in the middle of the highway. The source said there were nearly 100 vehicles around the campaign bus. Biden staffers were rattled by the event, the source said, though no one was hurt.