PITTSBURGH — An armed man took a hostage inside an upper floor of a downtown office building this morning, forcing police to evacuate the high-rise.

Police negotiators were in contact with the man after he took the hostage shortly after 8 a.m. on the 16th floor of Three Gateway Center, one of six office buildings that dominate the city skyline, Pittsburgh Police Chief Nate Harper said.

Authorities know the identities of the gunman and the hostage but were not releasing them, Harper said. He described the suspect as a black male in his 20s who is “ex-military” and has a criminal history. He said no injuries had been reported and no shots had been fired.

Harper said that police are in contact with the gunman, but are not in contact with the hostage.

At about 10:45 a.m., a woman who police identified as the gunman’s mother was talking to police.

In addition to city police, Allegheny County police, state police, FBI and ATF were at the scene.

The gunman and the hostage are in the offices of CW Breitsman Associates, a firm that runs employee-benefits programs.

Police arrived soon after and began evacuating the building.

Several hundred people milled around about a half-block from the 24-story building after police evacuated it. A light rail stop nearby was closed and at least 20 emergency vehicles surrounded the building.