WTOP and the AP are reporting that a man threw white powder of an unknown origin at a security guard in the Wilson Building at 10 a.m. this morning. A hazardous materials investigation has shut down traffic along Pennsylvania Ave. NW between 13th and 14th streets, and has closed the north entrance to the building.

Mark Segraves is reporting from the scene:

It’s believed the man who threw the substance then ran from the building.

Segraves reports the white powder dispersed in the outer lobby before the grand foyer of City Hall.

Building services, not thinking that the substance could be dangerous, then started to clean it up, taking it downstairs to a trash room before the fire department was called.

The fire department does not believe the substance is actually dangerous, according to Segraves.

UPDATE 2:29 p.m.: WJLA says the powder appears to be table salt.

Photo by Eye Captain