An armed man, who held three hostages in the Discovery Communications building in the US state of Maryland, was shot dead on Wednesday afternoon, police said.
All three hostages are safe, J. Thomas Manger, police chief of Maryland's Montgomery County, was quoted as saying by The Washington Post.
Law enforcement officials fired at the suspect, James J. Lee, because police "believed the hostages' lives were in danger," he said.
Police had been negotiating with Lee for several hours. Manger said Lee displayed "a wide range of emotions" during the talks.
One of the explosive devices the gunman had in his possession appeared to go off, Manger said.
The standoff began at 1 pm on Wednesday after the man walked into the large office building in the heart of downtown Silver Spring waving a handgun and wearing what appeared to be metallic canisters on his chest and back.
Manger said that most of the 1,900 people who work at the Discovery building were safely evacuated, including all of the children at the day-care centre located there.
Police said they have no reports of injuries.
The suspect was said to have a device on his body and was holding another device, both with blinking lights, police said.
A manifesto posted on a website registered to a person named James Lee, who gave a post office box in Canada as his address, lists several demands to the Discovery Channel, saying the station "must broadcast to the world their commitment to save the planet", the daily said.
It lists 11 demands about airing shows that would promote curbing the planet's population growth, finding solutions for global warming and dismantling "the dangerous US world economy".
"All programmes on Discovery Health-TLC must stop encouraging the birth of any more parasitic human infants and the false heroics behind those actions," the manifesto reads. The manifesto was published on http://SavethePlanetProtest.com.
Law enforcement sources said they believe the site was operated by the same person who was inside the building.