“He then began to drive forward, slowly hitting people,” the Facebook post read. “It was only once he was already hitting people and increasing his speed that people began to hit his car. He continued to drive faster, deeper into the crowd, turning to his left as he went.”

A witness said in an interview that the activists had hoped their vigil would ensure that the wider community knew about Ms. Herring’s death and about their questions over the police’s version of events.

After leaving the vigil, which was attended by about 100 people, the demonstrators chanted while walking through the streets. They stopped in the intersection seen in the video, and several cars turned around or found other routes when they saw the protesters there, said the witness, Jay-Marie Hill, 28.

When the car drove directly up to the activists, they perceived it as a threat, the witness said.

“People went around the car and said, ‘You need to turn around, this is a peaceful protest.’ ”

On Tuesday, at a news conference about Ms. Herring’s death, Lawrence O’Toole, the acting police commissioner of the St. Louis Metropolitan Police Department, said officers arrived at a home shortly after 8 a.m. to find a person had been “severely cut.” The person identified Ms. Herring as the assailant, he said.

When an officer went to take Ms. Herring into custody, she slashed at that officer with a kitchen-type knife, hitting him on the arm, Mr. O’Toole said. The officer was treated at a hospital for a minor injury.

Ms. Herring was a mother of three who was described by Nicholas Herring, a nephew, as “kind, caring and hilarious,” according to The Post-Dispatch.