UPDATE: A blog post from the scene.
ANOTHER UPDATE: More at the Mudville Gazette. Note that early reports are usually wrong, but it sounds quite bad, and likely to have some sort of terror connection. But stay tuned.
MORE: SayUncle: “FBI guy on Fox says no terrorist nexus. Probably means interview with guy in custody indicates it’s a civilian, contractor, or military guy. Preliminary reports are usually wrong. So, take the above with a grain of salt.”