UPDATE 10:07 - Looks like we have our answer, courtesy of the WaPo traffic reporter's blog:
dcfiremes just tweeted this: "update - PO @N Cap & Mass - DC F&EMS Haz Mat has determined scene is stable - no hazard - turning over to police - no sickness no transports"
Looks like we're all clear. (phew!)
UPDATE 9:55 - Per tier1express in the comments, there's a twitter report that another suspicious package has been found, different from yesterday's anthrax hoax at the APA. Will attempt to confirm.
UPDATE 9:40 - This appears does not appear to be leftover action from the suspicious package scare at the American Psychological Association last night, but the police were placing fresh tape on the scene and new vehicles were constantly arriving. There may be a new development that kept so many jurisdictions on-scene, and I'm not sure why Capitol Police would be involved to begin with. Still seeking info.
I haven't found any news about this, but there is a very large police/emergency services operation going on in DC right now. No idea if it's a drill or an actual event, and nothing about it is showing up on Google News yet.
The basics: when I got off the Metro at Union Station about a half-hour ago, there were around a dozen officers (and a K-9 unit) on the platform. Outside, Massachusetts Ave. is completely blocked off in front of the National Postal Museum, and about a block radius around it. I saw uniformed officers from at least three agencies, including Capitol Police (Capitol Police officers were redirecting pedestrians at Mass Ave) and what I suspect was a fourth agency in unmarked Suburbans blocking off part of New Jersey Ave.
Sirens are going as more vehicles arrive. There are several fire and emergency vehicles on scene. Someone told me secondhand that there were people on stretchers. (unverified)
Again, no details beyond that. I'll keep searching for information.