SILVER SPRING, Md. - Montgomery County Police have shot and killed a gunman who held several people hostage in the Discovery Communications building in Silver Spring for nearly four hours Wednesday.

Three people who had been taken hostage are now safe and out of the building, Montgomery County Police Chief Tom Manger says.

Manger says the suspect was shot at 4:48 p.m. Wednesday.

Manger says an explosive device may have detonated, and the suspect may have brought other devices into the building. Authorities believe there was some smoke and may have heard a popping noise but have not confirmed anything.

"We believe there are other suspected devices in the building that have not been rendered safe. The operation at this point is to make sure those devices are rendered safe and removed," Manger says.

WTOP's Adam Tuss and Kate Ryan - who are on the scene - reported hearing a popping noise shortly before 5 p.m.

Police had been in negotiations for nearly four hours over the phone with the gunman, who was on the first floor area of the building.

Manger would not confirm the identity of the suspect, or his demands.

Police say the man burst into the main lobby of the downtown Silver Spring office around 1 p.m. Wednesday.

"He was wearing what appear to be metallic canister devices on his front and back. He also pulled out a handgun and was waving the handgun around," Manger says.

The building has been evacuated. Most of the building's 1,900 employees are out, Manger says.

Police have not identified the suspect, but say he has "concerns" with the Discovery Corporation. Montgomery County Police Capt. Paul Starks would not say if the man works there now or worked there in the past.

WTOP National Security Correspondent J.J. Green reports the suspect's name is James J. Lee and he lives in the area.

Green reports Lee is 43 years old and has been involved in protests at the Discovery Communications building for several years.

Lee was arrested in 2008 for disorderly conduct and littering on the grounds of the building, according to police reports. At the time, Montgomery County Police said they were concerned with his mental state.

Sources tell Green the suspect may have a list of demands on the Internet.

Law enforcement sources tell WTOP the suspect may have a propane tank strapped to him and may be carrying a silver pistol.

Sources are saying the suspect is allegedly upset about the documentary called "Whale Wars."

A daycare with about 100 children inside the building was also evacuated. WTOP's Kate Ryan, who is on the scene, says children were being rolled out in cribs. The children were taken to a nearby McDonald's.

Multiple witnesses reported shots were fired, WTOP's Brennan Haselton reports.

A person inside the building told The Associated Press says the man had something strapped to his chest and has hostages. The person refused to be identified, saying people inside have been told not to speak to the media.

SWAT teams are at the building, which is located on Georgia Avenue in downtown Silver Spring.

Bomb technicians are also on the scene, Montgomery County fire department spokesman Capt. Oscar Garcia says. ATF and FBI officials are also on the scene.

Faisal Afzal, a systems engineer at Discovery, tells WTOP he was heading back into the building after lunch at about 1 p.m. and was about 20 feet from the door when he heard a street vendor yelling.

"We heard a street vendor yelling, there's a guy with a gun, there's a guy with a gun. He's in the building. And I saw people lying down on the floor at the building. And we just ran away from the building and we heard a couple shots fired," Afzal tells WTOP.

"I saw at least four people lying sideways on the floor."

Adam Dolan, a sales director in Discovery's education division, told The Associated Press by phone that he was heading to lunch with a co-worker when he heard there was a situation in the building.

He was told to go back up to the top floor, the ninth, and lock the door and turn off the lights. Eventually the workers were herded down a stairwell and told to go home.

"Everyone was very scared, but at the same time ... I think people were calm and collected and responded as one would expect in this situation," said Dolan, 28.

When he got to the bottom floor, he saw shattered glass near the company's daycare and suspected it was broken to get the children out.

Discovery employees tell WTOP a mass email was sent initially telling employees to shelter in place in their offices because there was a gunman in the lobby. Then, employees were told to move to the side of the building furthest away from the lobby side of the building. Employees were then moved to higher floors before being evacuated by through the stairwells.

WTOP's Kate Ryan, who is on the scene, says there is an amazing police presence in the area.

"Police have absolutely saturated the area," Ryan says.

Authorities descended on the area, and people are being kept away from the main drag of the downtown area where the building is located amid restaurants and shops.

Roads in the downtown Silver Spring area are closed. Drivers are being urged to avoid the area, if possible.

Georgia Avenue is closed in Montgomery County in both directions from Colesville Road to Wayne Avenue.

Traffic entering Silver Spring on northbound Georgia Avenue is being rerouted left on Spring Street and then right onto 16th Street. Southbound traffic can also use 16th Street as an alternate route.

DDOT is also encouraging northbound motorists on Georgia Avenue in the District to use Piney Branch Road as an alternate route to bypass the closures on Georgia Avenue. Eastern Avenue, on the District/Montgomery county border, can also be used to get to 16th Street.

The situation is not affecting Metro rail service. However, the Metro entrance on Wayne Avenue is closed, WTOP's Adam Tuss, who is on the scene, reports.

Metrobus routes are being affected.

St. Michael's School on Wayne Avenue is under lockdown. All students are accounted for, but will not be released until the situation is resolved.

Discovery Communications Inc. operates cable and satellite networks in the U.S., including The Discovery Channel, TLC and Animal Planet. Discovery shows include "Cash Cab" and "Man vs. Wild," and TLC airs "American Chopper" and "Kate Plus Eight."

Animal Planet also airs the controversial series "Whale Wars," about attempts by environmentalists to disrupt the Japanese whaling industry.

(Copyright 2010 by WTOP and The Associated Press. All Rights Reserved.)