A spokesman for the senator says the office furloughed most staffers after receiving advice from the Senate Employment Counsel about the Anti-Deficiency Act, which says its unlawful to spend government funds during a shutdown.
Only five staffers remain, out of 51.
"I wanted to make sure my staff was not treated any differently than other federal workers who have been furloughed," McCaskill said. "I had the ability to say the rules don't apply to our office, but most of Missouri's federal workers don't have that luxury, so I made a decision to cut it to the bare bones."
Calls to her office are now being picked up by an answering machine.
Missouri's other Senator, Republican Roy Blunt, has left his office open during the shutdown.