Update at 11:50 a.m. — All lanes are now back open.
Update at 11:35 a.m. — The Key Bridge is being reopened, D.C. police said via Twitter. Traffic is currently flowing from the D.C. to Virginia side of the bridge, but so far the inbound lanes have not reopened. NBC Washington is reporting that the closure was due to a phoned-in bomb threat.
Earlier: Police have closed the Key Bridge to vehicle and pedestrian traffic due to police activity on the D.C. side.