Police in St. Louis County said two young males were injured in what appears to have been a drive-by shooting near a monument to Michael Brown, who was shot to death a year ago.

Spokesman Shawn McGuire said in an email that the 17-year-old and 19-year-old males told officers they were walking at around 2:15 a.m. Monday near the Canfield Apartments in Ferguson, Missouri, when an unknown man shot at them from the back of a passing vehicle.

McGuire said that the 17-year-old was shot in the chest and shoulder and the 19-year-old was shot in the chest. He said both were being treated at a hospital and that neither had life-threatening injuries.

The shootings come as the St. Louis suburb marks the anniversary of the killing of Brown, a black 18-year-old, by a white Ferguson police officer. The anniversary was punctuated by a shooting.

Earlier, St. Louis County Police Chief Jon Belmar said a man who opened fire on officers in Ferguson was in "critical, unstable" condition after being struck when the officers returned fire.

Belmar said at a news conference early Monday morning that plainclothes officers had been tracking the man, who they believed was armed, during a protest marking the anniversary of the death of Brown.

Belmar said the man approached the officers, who were in an unmarked police van, and opened fire. The officers returned fire from inside the vehicle. They then pursued the man on foot.

The chief said the man again fired on the officers. All four fired back.

The man was taken to a hospital, where Belmar said he was in surgery. The man wasn't immediately identified.

Belmar said the police shooting happened shortly after dozens of shots rang out around 11:15 p.m. Sunday, as protesters were gathered on West Florissant Avenue. The crowd went running for cover.