SILVER SPRING, Md. — Police shot and killed a gunman wearing explosives after he took three people hostage at the Discovery Channel’s headquarters Wednesday afternoon, officials said.

A spokesman for the company said it planned to reopen Thursday, though the building remained a crime scene Wednesday evening as authorities searched for explosives that might have been left by the gunman.

Authorities identified the gunman as James J. Lee. Lee, 43, who was upset with the channel over its programming and had a history of protesting the company, entered the building about 1 p.m., wielding a gun and wearing silver canisters later described as “”explosive devices,”” and “”told everyone to stop moving,”” according to police. Most of the 1,900 employees were able to escape, but Lee managed to grab three male hostages — an unarmed security guard and two Discovery employees.

Tactical officers were able to watch Lee’s movements via cameras. After several hours of telephone negotiations, Lee pulled out his gun and pointed it at one of the hostages, police said. Officers who had been tracking him then took aim at Lee, killing him. Police said there was an explosion at the time of the shooting, but it was unclear whether Lee was shot first or if the device detonated first.

All three captives escaped safely. Police said they don’t know what precipitated Lee at that moment. Saying only that the hostages “”appeared to be moving”” and that might have agitated him.

Lee had staged a protest outside Discovery headquarters in 2008 and had a website that ranted against the channel’s programming.

“”I know that he had some history with folks at Discovery Channel,”” Montgomery County Police Chief J. Thomas Manger said at a news conference after the shooting.

David Leavy, a spokesman for Discovery Communications, said, “”He didn’t think we were environmentally sound,”” adding, “”there had not been any communication from him in the last couple of years.””

According to electronic court records, Lee was charged with disorderly conduct in 2008 and sentenced to 46 days in jail for a protest he staged in front of the channel’s headquarters. County State’s Attorney John McCarthy said Lee was ordered to stay 500 feet away from Discovery headquarters as part of his probation, which ended two weeks ago. Lee was found not guilty of the lesser charge of littering during the same incident when he threw thousands of dollars in the air.

“”It was a very odd case; he had large sums of cash on him,”” McCarthy said Wednesday.

McCarthy said that District Administrative Judge Eugene Wolfe ordered a psychiatric evaluation for Lee, but the results did not affect his probation. He said it was unclear where Lee got the money for the protest.

Lee said he protested to show that “”money means nothing. Money is trash,”” according to a report by The Gazette newspaper at the time of the incident. Lee also said he was protesting that Discovery’s programming had little to do with saving the planet.

“”He was steadfast, he was adamant about what he was doing. It was illogical,”” said Felix Contreras, a Silver Spring resident who met Lee in 2008.

He said Lee was trying to recruit people to picket out in front of the company’s headquarters when he met him. Contreras said Lee offered him $100 to walk around with a sign.

Leavy, the Discovery spokesman, said Wednesday that the company’s priority is “”responding to employees’ needs in the coming days.””

Police and officers with the Montgomery County bomb squad remained on the scene Wednesday night, checking for more explosive devices. Battalion Chief Kevin Frazier of the bomb squad said Lee had two boxes and two backpacks that they believe contained explosive devices.

Annie Frank, a web content publisher at Discovery, said by phone that she was at her desk when the incident unfolded.

“”I was actually out where the gunman was, 40 minutes before this all went down, talking to my sister,”” she said. “”I went back up to floor 5, my cubicle. I look out the window, and there’s all these cops.””

Frank said her boss received a phone call warning that there was a gunman in the building and telling them to get into an office. She said they went into the nearest office, closed the door and shut off the lights.

Protesters had tampered with phones in their building a few weeks before, Frank said, so the threat initially wasn’t taken too seriously.

“”We didn’t really grasp what was going on,”” she said. “”We thought someone was coming in with a toy gun, protesting or something. And then we realized it wasn’t being contained.””

Still shaken up, Frank, 26, said she couldn’t estimate how long she and her co-workers stayed in the office.

Frank said a security guard eventually escorted them to an adjacent building, where they waited for another 15 minutes before they were told to run to an exit staircase and get to their cars if they could.

Frank said they were told “”don’t stop, just go.””

Jessica Kim, 22, who works in the building, had gone to lunch with a co-worker and “”by the time we got back, the building was surrounded by police.””

Road closures brought the normally bustling business district to a standstill.

“”I thought it was a joke at first,”” Kim said. “”I’ve never seen anything like that in my life.””

Over the next hour, she said she received more than 50 calls, texts and e-mails from concerned family and friends.