FERGUSON, Mo. — A man who authorities said 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 today.

Chief Jon Belmar said at a news conference that officers had been tracking the man, 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 man approached the plainclothes officers, who were in an unmarked police van, and opened fire, Belmar said. The officers returned fire from inside the vehicle and then pursued the man on foot when he ran.

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

The man was taken to a hospital, where Belmar said he was in “critical, unstable” condition. Authorities didn’t immediately release the identities of anyone involved.