UPDATE (5:05 p.m.): The Discovery Channel suspect has been shot and the hostages are safe. Police officers shot the suspect at about 4:50 p.m., Montgomery County Police Chief J. Thomas Manger told reporters. All three hostages are safe and out of the building. "The suspect is in custody," he said. Manger said he had no information about the suspect's condition. A number of devices in backpacks have to be rendered safe, he added.
CNN is reporting that the Discovery Channel in Silver Spring, Md., sent an e-mail to all employees in the building at 1:21 p.m. telling them it has "reason to believe there is an armed gunman at One Discovery Place." It is also believed that the gunman is in possession of an explosive device and has taken at least one person hostage. The SWAT team and the fire marshals with explosive experience have arrived. The Discovery Channel Daycare has been evacuated. This story is developing.
Check back for updates.