Police say the man entered the building sometime after 8 a.m. Friday and went up to a 16th floor office suite inside Three Gateway Center.
Chief Nate Harper says the man took at least one hostage — possibly two hostages — inside a business after entering the suite and asking about a particular person.
A witness who works on the 16th floor identified the business where the hostage situation was unfolding as CW Breitsman Associates, a firm that helps businesses run 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 the 24-story building after police evacuated it.