Photo courtesy Chet Susslin of National Journal.

UPDATE (3:44 p.m.): The Post now reports that the Capitol Police did not recover a weapon after apprehending the man, which raises the question of why, precisely, the man was detained in such a dramatic manner. Obviously, we’ll keep an eye on this story as it develops. We have also altered the headline of this post to remove the reference to a gun until more information is released.

So what exactly caused the U.S. Capitol to be locked down this morning? U.S. Capitol Police apprehended an unidentified man who was allegedly brandishing a gun at the intersection of First Street and Constitution Avenue NE, which is near the Senate side of the Capitol’s offices. It wasn’t clear whether the man was threatening anyone.

Chet Susslin at National Journal has the best photographs we’ve seen so far of the incident — Susslin just happened to be walking by the Capitol when the whole thing went down.