Let's see:
Multiple witness descriptions of more than one middle eastern male cited.
Multiple witness descriptions of white vans/trucks as escape vehicles.
Victims include men, women, children in the metro DC area.
Rifle shots to the head/body.
Tapes and documents from al-queda describe an interest in gunning down Americans.
No killings on the weekends -- synchronizing to the media cycle?
US Law Enforcement: Can't use the "T" word!!!
Conclusion: Must be the Montana Militia.