At least one hostage has been taken by an armed man inside the Discovery Communications headquarters in Silver Spring, Maryland, police say.
The gunman, named by US media as James Lee, had metallic canisters strapped to his chest and a handgun when he entered the building, police said.
Montgomery County Police Captain Paul Starks said officers were communicating with the man.
The area around the building, outside Washington DC, has been sealed off.
Discovery employees were evacuated from the building shortly after the incident began at around 1800GMT.
But police said some of the 1,900 workers could still be inside the building.
One female Discovery worker told the BBC that two hostages were being held inside the building.
Ongoing hostage situation
Montgomery County Police Chief J Thomas Manger said a man entered the Discovery headquarters through the main entrance wearing what appeared to be "metallic canisters".
The man then pulled out a handgun and told everyone to remain still.
"We are in negotiations with him currently," Mr Manger said.
Captain Oscar Garcia of the Montgomery County fire department said bomb technicians were at the scene of the incident.
US media named the gunman as James Jay Lee, a man in his 40s known for protesting outside the building.
Discovery Communications reaches 1.5bn subscribers in 180 countries through networks like Discovery Channel, Animal Planet, and the Science Channel. source