(CNN) Two people were killed and a police officer was wounded in a Monday morning shooting at a methadone clinic in Baltimore, city Police Commissioner Michael Harrison said.

Police responded at 7 a.m. to a call about a man armed with a gun at the Man Alive treatment center on the 2100 block of Maryland Ave, Harrison said at a morning press conference.

People standing outside the clinic informed arriving officers that a man inside had recently fired shots. Treating the incident as an active shooter scenario, officers entered the facility and attempted to de-escalate the situation a number of times, Harrison said.

The suspect, who was not named, began to fire at the police officers, who then returned his fire, Harrison said.

Baltimore Police Sergeant Bill Shiflett was struck by shots fired by the suspect, Harrison said.

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