A man fired a gun into a crowd in downtown Denver at around 1 a.m. Sunday morning. An off-duty police officer and a security guard returned fire, critically injuring the gunman and someone who was with him, according to the Denver Police.

The shooting seemed to arise from a confrontation near Auraria Student Lofts, although no one involved seemed to have a connection to the building, police said.

A guard for the building tried to break up the confrontation with some kind of chemical agent, such as pepper spray, according to police. The guard signaled with a flashlight for assistance from the officers, who were working nearby.

At some point, one of the groups involved in the confrontation walked away, but two men then began walking back toward the crowd. One, a 40-year-old man, drew a pistol and fired multiple gunshots toward the crowd.

An officer and the guard returned fire, striking the man and another 35-year-old man, who may be related to the suspect. The shooter may be charged with aggravated assault or attempted murder, but no charges were expected for the second wounded man.

Both men are expected to survive.