Senator Claire McCaskill reports on Twitter that shots have been fired outside the Capitol:

Shots fired outside the Capitol. We are in temporary lock down.

— Claire McCaskill (@clairecmc)
October 3, 2013

One House member says the "shooter has been arrested."

I am told shooter has been arrested. House floor activity stopped for now.

— Tim Griffin (@TG4Arkansas)
October 3, 2013

And there's this:

The Capitol is on lockdown pic.twitter.com/SvTD5GdKcd via @learyreports

— HuffPost Politics (@HuffPostPol)
October 3, 2013

What's being displayed on Capitol TVs right now pic.twitter.com/dZVtYCW5iJ (via @mmcauliff)

— Judd Legum (@JuddLegum)
October 3, 2013