PITTSBURGH (AP) — An armed man took a hostage inside an upper floor of a downtown office building on Friday, 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 “ex-military,” but didn’t elaborate. He said no injuries had been reported and no shots had been fired.

A worker on the same floor said the hostage-taking unfolded inside the offices of CW Breitsman Associates, a firm that runs employee-benefits programs.

Kathi Dvorak, an administrative assistant at AXA Advisors, said a woman ran into her office yelling for someone to call 911. A second woman ran in and said her office was being robbed.

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.

Joel Kirchartz, a 28-year-old web developer who works on the 17th floor, said he and his co-workers looked out the windows Friday morning and “a bunch of cops pulled up with all sorts of sirens going; there must have been 20 of them.” He said he went downstairs to find out what was happening and by the time he got outside, police had sealed the building.

Another worker, Sarah Vereb, said she was at her desk when she was ordered to leave the building shortly after a friend called to report that she wasn’t being allowed up from the lobby.

Hundreds of workers walked down the stairwell. Vereb said the exodus was orderly and “very, very quiet.”

A phone message left for the building’s Santa Monica, Calif.-based ownership group was not immediately returned. The building complex management office confirmed the evacuation and said it was working with police.