Malathi Kanche was heading home after dropping her son off at college Wednesday evening when the small S.U.V. she was driving was overwhelmed by floodwaters set off by the remnants of Hurricane Ida.

With the vehicle stalled in waist-deep water on Route 22 in Bridgewater, N.J., she and her 15-year-old daughter climbed out. They clung to a tree as the torrent rushed past, according to a close family friend and neighbor, Mansi Mago.

Then the tree gave way, and “the water took her,” said Ms. Mago, recounting what another stranded motorist told her hours later.

A 46-year-old software designer who emigrated from India, Ms. Kanche was one of six people who were still missing two days after Ida caused the deaths of at least 25 people in New Jersey — more fatalities than in any other state — as the monster storm whipped its way onto the Gulf Coast and tore north to New England.

At least a third of the fatalities in New Jersey were people who drowned after being trapped in vehicles in a densely packed state known for its car culture, its tangle of highways, suburban commuter towns and limited public transportation.