FERGUSON, Mo. — A suspect who authorities say opened fire on officers in Ferguson, Missouri, on the anniversary of Michael Brown’s death was critically wounded when the officers shot back, St. Louis County’s police chief said early Monday.

Chief Jon Belmar said at a news conference that officers had been tracking the suspect, who they believed was armed, during a protest marking the death of Brown, the black, unarmed 18-year-old whose killing by a white Ferguson police officer touched off a national “Black Lives Matter” movement.

The suspect approached the plainclothes officers, who were in an unmarked police van, and opened fire, Belmar said. The officers shot back at him from inside the vehicle and then pursued him on foot when he ran.

The suspect again fired on the officers, the chief said, and all four officers fired back. He was struck and fell.

The suspect was taken to a hospital, where Belmar said he was in “critical, unstable” condition. Authorities didn’t immediately release the identities of anyone involved, but Tyrone Harris told the St. Louis Post-Dispatch that the injured suspect was his son, 18-year-old Tyrone Harris Jr.

The elder Harris told the newspaper shortly after 3 a.m. that his son had just gotten out of surgery.

None of the officers was seriously injured. All four have been put on standard administrative leave. They were not wearing body cameras, Belmar said.

The shooting happened shortly after a separate incident that the chief called “an exchange of gunfire between two groups” rang out around 11:15 p.m. Sunday while protesters were gathered on West Florissant Avenue, a business zone that saw rioting and looting last year after Brown’s killing. The shots sent protesters and reporters running for cover.

The chief said an estimated six shooters unleashed a “remarkable” amount of gunfire over about 45 seconds.

Belmar waved off any notion that the people with the weapons were part of the protest.

“They were criminals. They weren’t protesters,” he said.

The suspect who fired on officers had a semi-automatic 9 mm gun that was stolen last year from Cape Girardeau, Missouri, according to the chief.

“There is a small group of people out there that are intent on making sure that peace doesn’t prevail,” he said. “There are a lot of emotions. I get it. But we can’t sustain this as we move forward.”

Some protest groups were critical of police.

“It was a poor decision to use plainclothes officers in a protest setting because it made it difficult for people to identify police officers, which is essential to the safety of community members,” Kayla Reed, a field organizer with the Organization of Black Struggle, said in a statement.

“After a year of protest and conversation around police accountability, having plainclothes officers without body cameras and proper identification in the protest setting leaves us with only the officer’s account of the incident, which is clearly problematic.”