A man known for protesting the Discovery Channel's environmental programming stormed the network's Maryland headquarters carrying a handgun on Wednesday, holding three people hostage for hours until he was shot and killed by police, according to authorities.

The hostages -- two Discovery Communications employees and a security guard -- were unhurt after the four-hour standoff. Montgomery County Police Chief Thomas Manger said tactical officers moved in after officers monitoring the man on building security cameras saw him pull out a handgun and point it at a hostage.

The gunman was identified by Discovery Communications as James Jay Lee of San Diego, Calif., a man well-known to the building's employees because of his history of protesting the network.

Manger said the gunman had been wearing "what appeared to be metallic canister devices" when he entered the building.

"The man told everyone to just stay still," Manger said.

A website registered to Lee criticized Discovery and announced plans for the protest in January 2008: "These guys have been very sneaky and deceptive as to their contribution to the planetary problems. Just look at their 'new' show about saving the planet, 'Planet Green,' to me, it's just another show about more PRODUCTS to make MONEY, not about actual solutions. We can't let them get away with doing it anymore."

Lee is believed to have distributed a manifesto outside the Discovery building several weeks ago that called on the network to "broadcast to the world their commitment to save the planet."

"All programs on Discovery Health-TLC must stop encouraging the birth of any more parasitic human infants and the false heroics behind those actions," the list of demands read.

"In those programs' places, programs encouraging human sterilization and infertility must be pushed. All former pro-birth programs must now push in the direction of stopping human birth, not encouraging it."

Some witnesses reported that the man fired a shot before declaring, "Nobody is going anywhere," police said.

Employees at the Discovery Channel building were told to take cover in locked offices, according to local press accounts.

A company e-mail sent to employees reportedly said: "Don't Return to 1DP Until Further Notice...No employee should return to 1DP for any reason."

Manger said most of the 1,900 people who work in the building were able to get out safely while the hostage standoff was unfolding.

Witnesses from a building across the street told Fox News that authorities rushed four stretchers into the building.

Children at a day care center located inside the Discovery Building were reportedly safely evacuated to a nearby McDonald's restaurant. Some babies were wheeled out of the building while still in their cribs.

Police sources have reportedly said that Lee previously protested outside the building, where he was arrested and charged with disorderly conduct in February 2008, according to court records.

Police reports indicate he paid homeless people to join his protest and carry signs outside the building. He gave one individual $1,000 for what he considered a prize winning essay.

At one point, a crowd of more than 100 people gathered around Lee, who referred to money as "just trash" and began throwing fistfuls of it into the air.

At the trial, Lee reportedly said he began working to save the planet after being laid off from his job in San Diego. He said he was inspired by "Ishmael," a novel by environmentalist Daniel Quinn and by former Vice President Al Gore's documentary "An Inconvenient Truth."

Thomas Leonard, Lee's brother-in-law, told Fox News that Lee became a "darker type of character" after some deaths in the family and said he believed Lee was capable of killing.

Leonard described Lee as "talented but misguided" and said his "spirit took a nose-dive" in recent months.

"He's been very hurtful to loved ones who have tried to help him over the years," Leonard said.

Discovery Communications LLC 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.

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The Associated Press contributed to this report