Meave Ryan saw the boy waving his arms as she braked to a stop behind a string of cars. The other cars went on through the intersection, but she rolled down the window. “He was screaming for help,” she said. “He said, ‘My mommy just drove the car in the water.’ ”

Ms. Ryan, 31, told the boy — La’Shaun Armstrong, 10 — to hop in. She drove the short distance to the boat ramp where he said his mother had driven into the Hudson River in Newburgh, N.Y. “I got out of the car and went halfway in the water to see if I could see their car,” Ms. Ryan said.

The vehicle, a black minivan, was already submerged. Ms. Ryan told La’Shaun to get back in her car, and they drove to Fire Department headquarters a few blocks away. It was all but empty; the fire trucks were out on a call. “We knocked on the dispatcher’s door,” Ms. Ryan said. It was 7:50 p.m. on Tuesday.

It did not take long for the horrifying meaning of the boy’s story to register: His mother, Lashanda Armstrong, 25, had done what La’Shaun had said — she had, in fact, driven the minivan into the river. La’Shaun had escaped, swimming through 45-degree water.