Police say the man entered the high-rise sometime after 8 a.m. Friday and went up to the 16th floor.
Chief Nate Harper says the man took one hostage inside a business after entering an office suite and asking about a particular person.
A witness who works on the 16th floor identified the business as CW Breitsman Associates, a firm that runs employee-benefits programs.
Harper says no shots have been fired and no injuries are reported.
Several hundred people milled around about a half-block from Three Gateway Center, a 24-story building, after police evacuated it.