CLAYTON, Mo. — A 20-year-old man charged Sunday with shooting two police officers watching over a demonstration outside the Ferguson Police Department had attended a protest there earlier that night but told investigators he wasn’t targeting the officers, authorities said.

St. Louis County Prosecutor Robert McCulloch said Jeffrey Williams told authorities he was firing at someone with whom he was in a dispute.

“We’re not sure we completely buy that part of it,” McCulloch said, adding that there might have been other people in a vehicle Williams is accused of firing from.

Williams is charged with two counts of first-degree assault, one count of firing a weapon from a vehicle and three counts of armed criminal action.

The officers were shot early Thursday as a late-night demonstration began to break up after the resignation of Ferguson Police Chief Tom Jackson following a Justice Department report that found widespread racial bias in the police department.

“He was out there earlier that evening as part of the demonstration,” McCulloch said.

But several activists who have been involved in the protests since the Aug. 9 fatal shooting of 18-year-old Michael Brown by a Ferguson police officer said they were not familiar with Williams.

Williams used a handgun that matches the shell casings at the scene, McCulloch said. He also said tips from the public led to the arrest.

Williams, who St. Louis County Police Chief Jon Belmar said is black, is being held on $300,000 bond.

Brittany Ferrell, 26, a protest leader with the group Millennial Activists United, had just left a meeting with other leaders Sunday when word of the arrest circulated.

She said no one in the group knew Williams, and they checked with other frequent protesters – who also hadn’t heard of him.

Ferrell said she suspected McCulloch tried to cast him as a protester to reflect negatively on the movement.

“This is a fear tactic,” she said. “We are very tight-knit. We know each other by face if not by name, and we’ve never seen this person before.”

John Gaskin, a St. Louis NAACP leader, said of Williams, “I don’t know him. I’ve never seen him.”