"We have reason to believe there is an armed gunman at One Discovery Place. All employees should seek protection in a locked office on their respective floors immediately," read an email sent to employees and read over the public address system.
"There's a guy with a gun in the lobby. Police are swarming in -- assault rifles and all," one producer told ABCNews.com via instant message.
Employees at the building were told to take cover in locked offices and police have sent SWAT teams to the area to seal off nearby roads.The FBI and agents from ATF, including explosives experts, also rushed to the building.
Many were trapped inside the suburban headquarters.
"Friends, I am in work closet with workers here. There is a shooter here at Discovery. I am ok," read a text from one Discovery employee.
Another employee said the Lee had taken a hostage. "The police are trying to get the situation under control!INSANE!" read the email to ABCNews.
Some employees were initially sent to higher floors and later evacuated, police said. Witnesses on the scene said another group of employees left the building around 1:44 p.m.
The building has a daycare center, which was evacuated with infants rolled through the street in cribs.
"Crazy to see the babies coming out of childcare center piled into rolling cribs into the McD's across the street, a safe zone," said a witness Wednesday via email.
The building, which houses the offices of Discovery Channel and its sibling networks including Animal Planet, is located just outside Washington DC.
Lee operated the Web site savetheplanetprotest.com and was arrested in 2008 following a demonstration held at Discovery's headquarters. Lee was found guilty of disorderly conduct.
According to a post on his Web site, Lee said he was thrown "in the nut house" for four days following his arrest.
Lee has been demanding a boycott of Discovery Channel since at least 2008.
On his website, Lee posted a rambling manifesto under the title "My Demands," which espouses a far-radical environmentalist and misinthropic philosophy and calls on the channel to cease programming about giving birth, war and weapons.
"The planet," he wrote, "does not need humans."
The document, which appears to have been created on July 17, is interspersed with references to esoteric philosophers, childish language, misspellings, and capital letters.
Lee writes that the channel should cease its current programming and replace it with a game show about reducing the global population.
The channel, he wrote, should produce a program about "how people can live WITHOUT giving birth to more filthy human children since those new additions continue pollution and are pollution."
"The world needs TV shows that DEVELOP solutions to the problems that humans are causing, not stupify the people into destroying the world. Not encouraging them to breed more environmentally harmful humans," he wrote.
"Saving the environment and the remaning [sic] species diversity of the planet is now your mindset. Nothing is more important than saving them. The Lions, Tigers, Giraffes, Elephants, Froggies, Turtles, Apes, Raccoons, Beetles, Ants, Sharks, Bears, and, of course, the Squirrels. . . The humans? The planet does not need humans," he wrote.
ABC News' Matt Jaffe, Devin Dwyer and Ryan Thomas contributed to this report