Capitol Police just sent out this alert to Senate offices, and seem to be downplaying the level of the threat:
"Several offices have received a bomb threat by phone and email. The United States Capitol Police are actively investigating. There is no need for staff to take any precautions at this point. If your office receives a threat please take note of the time, caller I.D., and wording."
One Senate aide who alerted Politico to the bomb threats says all three Senate office buildings -- Dirksen, Hart and Russell -- have received the calls. It's not clear which offices received threats and exactly what the callers said when they called in the bomb threat.