There's no official police statements about who the gunman is or what he wants, but there's a list of demands circulating on Twitter—said to be the gunman's manifesto. (Edit: The original site is now down, I'm linking to a version one of our readers saved.) I'm not sure what the original source of this list is, yet, but I'll let you know if/when I find out. The demands are centered around a proposed new programming lineup for The Discovery Channel, pushing Malthusian ideas on population and Daniel Quinn's "My Ishmael" pages 207-212. Seriously. This would be almost funny if it weren't for the whole hostage thing.
The Discovery Communications building is being evacuated. Here's hoping everybody makes it out safely.
UPDATE: According to DCist, the list of demands apparently dates to 2008, and is linked to James Jay Lee, a man arrested that same year for disorderly conduct while protesting at the Discovery Communications building. No one has any idea who the gunman is at this point, so it's unknown whether this is the same guy. WUSA9 TV in Washington D.C. first tweeted the old demands as being linked to the current gunman, but it's unclear how they came to that conclusion.
For now, let's assume they aren't linked. It's worth noting that Discovery Networks have received a lot of threats recently because of Animal Planet's Whale Wars show, which follows the Sea Shepherd Conservation Society as it attempts to deter Japanese whaling ships.
UPDATE #2: Law enforcement officials are now saying that they believe the gunman is that 2008 protester, James Jay Lee.
UPDATE #3: Police have shot and killed Lee and freed the hostages.
Image of gunman taken by a colleague of Discovery employee Jason Divenere That colleague is now safely out of the building.