Based on everything reported in the press....this is a inside job...likely to be a former locksmith for the building. The fact that every single door in the Pentagon is run by scanning security cards to enter, and every door in this Washington DC office is plain old-fashion key-locks....makes this more of a joke. The congressional building should have upgraded a decade ago, and is still unsecure today.
I have one prime suspect: Eric Holder.
There is no possible way that a correctly secured Capitol office could be burgled without inside help, assistance or knowledge.