A gunman is holding a number of hostages inside the Discovery Channel building in suburban Washington, police confirmed tonight.
The man, who is understood to be of Asian descent, may have explosives strapped to his body, police said.
Police sealed off the area around the building and SWAT teams were at the scene. The incident began around 1pm local time. The building was evacuated and children were removed from a day-care centre in the building.
A police spokesman said the building in Silver Spring, Maryland, was on lockdown but he could not confirm gunshots were fired as reported by ABC Television.
"He appears to have some sort of explosive device on his person," a police spokesman told reporters. He said the police were in contact with the suspect, who had made some demands that police would not elaborate on.
"I don't know anything about his history," he added when asked by reporters about the suspect's possible relationship to Discovery.
MSNBC reported that police said the man's name was James J. Lee. A man named James Lee of San Diego was arrested in February 2008 after throwing money into the air in a protest outside the Discovery building, the Montgomery County Gazette had reported that year.
Police in nearby Takoma Park, Maryland, sent out an email to residents that said, "an unknown, armed Asian male is barricaded in the building with at least one hostage."
Montgomery county police corporal Dan Friz said of the suspect: "His concerns are with the Discovery Corporation right now." He would not go into further details.
"There may be some other potential devices with him, or near him that he brought in, but again none of this has been confirmed," he said to reporters, televised on local CBS.
A reporter from the local ABC affiliate said he saw an ambulance crew wheel a gurney into the building about an hour after the incident began, but there was no official word of casualties.
Police were watching the suspect on the building's closed-circuit security system and Discovery employees posted updates on the situation on Twitter including a link to a website, http://savetheplanetprotest.com/, in which demands are made on Discovery. It was not confirmed that these demands were made by the man barricaded in the building.
Roads in the normally busy suburban centre at the intersection of Colesville Road and Georgia Avenue on the edge of Washington were blocked to all traffic.