This started popping up on Twitter a few minutes ago, and now Fox News is reporting there's a full SWAT team at the scene. The Washington Post has a short write-up with the scant details:
A man with what appears to be an explosive device on him has taken at least one hostage at the Discovery Building in downtown Silver Spring, Montgomery County Police said.
Cpl. Dan Friz said police received a call about 1 p.m., of shots fired in the lobby.
Tactical officers are on the scene and developing a plan to deal with the situation.
Police initially said the building was being evacuated, but employees said they were told via email to stay at their desks.
Officers have not fired their weapons, Friz said.
The building has been evacuated, but there are also conflicting reports of some employees still sheltering in offices. The police cannot confirm shots fired by the suspect, but several people inside the building report hearing shots fired.
A man entered the lobby and may have fired a weapon and declared, "Nobody is going anywhere," Montgomery County police said.
In a phone interview on ABC 7 News and TBD TV, Discovery Communication employee Lisa Lucas said the building is in lockdown, with an unknown number of employees sheltering in offices. Police have established a cordon around the building.
She said that about 1:20 p.m., "we heard, Pop, pop, pop, pop, pop."
The building is the corporate headquarters of Discovery Communications, which owns the Travel, Discovery, TLC, and Military channels, among others.
Update: The police are saying the suspect is an Asian male. Fox has a source inside the building saying there is more than one hostage.
Update: Local news Channel 9, WUSA reports on its Twitter account that the gunman's demands can be found here, at www.savetheplanetprotest.com. They don't explain how they came to know that, but it's a very disjointed wacko environmentalist, anti-military, overpopulation alarmism rant.
Update: If it's the Save the Planet Protest guy, here's a DCist blog post poking fun at him from 2008. His name is Lee, and this has been a long-time crusade, it seems.