SILVER SPRING, Maryland (UPDATE) - A radical environmentalist took three men hostage in a dramatic siege at the headquarters of a US television channel on Wednesday before being shot and killed by police.

All the hostages were unharmed after the four-hour standoff at the Discovery Channel in which suspected explosives strapped to the gunman's body apparently detonated in a cloud of smoke.

Police said SWAT teams had fired on the suspect, who was under video surveillance during hours of tense negotiations, when he pointed his gun at one of the hostages.

"At that point our tactical units moved in. They shot the suspect. The suspect is deceased," police chief J. Thomas Manger told reporters.

The suspect was reported to be James Lee, an Asian American with a history of protest against the Discovery Channel, who was convicted in March 2008 of disorderly conduct, given six months of supervised probation and a small fine.

Manger acknowledged the gunman "had some history with the folks at Discovery Channel and I believe in fact that he was arrested here a couple years ago."

Lee entered the lobby of the sprawling building at about 1:00 pm (1700 GMT), waving a handgun with a package and "metallic" canisters strapped to his body, police said.

Bomb disposal experts were rushed to the scene, as the building and the surrounding area were evacuated. Infants were wheeled away in cribs from a day care center on the first floor of the building.

Police were still investigating reports the gunman fired at least one shot as he stormed into the building, a large complex in the Washington suburb of Silver Spring where some 1,900 people work.

Lee, who had his own Save The Planet protest site, admitted on a web page attributed to him he had a gripe with the Discovery Channel, which is known for its wildlife documentaries as well as military and survivalist programs.

He had protested outside their offices in the past, and his list of demands included that the cable station "broadcast to the world their commitment to save the planet."

Lee insisted that Discovery "stop encouraging the birth of any more parasitic human infants," cease promoting war and weapons of mass destruction through its technology programming, and promote an end to all immigration.

"Civilization must be exposed for the filth it is. That, and all its disgusting religious-cultural roots and greed," he said in one rant.

"Broadcast this message until the pollution in the planet is reversed and the human population goes down! This is your obligation. If you think it isn't, then get hell off the planet!"

Unchecked population growth was a theme attacked time and time again in Lee's postings.

"Saving the Planet means saving what's left of the non-human Wildlife by decreasing the Human population," he said. "That means stopping the human race from breeding any more disgusting human babies!"

Discovery Channel spokesman David Leavy said the station's security teams were familiar with Lee but had not considered him a threat because his views were so outrageous.

"If you follow what he said public online, I don't think it is rational," Levi said. "We certainly were cognizant and aware of him, but we did not take his demands or threats seriously."