US police shot and killed a man who took three people hostage, waving a gun and apparently fitted out with explosives, in the headquarters of the Discovery Channel near Washington Wednesday.   

Officers who had been watching the hostage drama on a building security camera crept in while police negotiated with the emotional gunman and shot him when he pointed his pistol at one of the three men he held hostage.

"A hostage moved, he pulled his gun, and a shot was taken,'' Montgomery County police chief Tom Manger told reporters. He said the suspect was killed and the hostages were safe.

The man, named by a US law enforcement official as James Lee, had been arrested before for protesting against Discovery Channel over environmental issues.

"He had a history ... of conflict with Discovery,'' Manger said. The incident caused chaos in Silver Spring, Maryland, a shopping and office district and commuter hub on the edge of the US capital.   

Police sealed off the area around the building and SWAT teams deployed shortly after the suspect entered the building carrying a handgun at about 1 pm. Manger said county and state police, FBI and Homeland Security agents joined the operation.   

The building, where nearly 1,900 employees work, was evacuated and children were rushed from a day-care center.

Police said they were still trying to determine whether two boxes and two backpacks that the hostage-taker had were explosive devices. Bomb-sniffing dogs checked the area before workers were allowed to leave neighboring buildings. - Reuters