WASHINGTON — Interest rates on short-term Treasury bills were mixed in Monday’s auction with rates on three-month bills falling to the lowest level in four weeks while rates on six-month bills were unchanged.

The Treasury Department auctioned $32 billion in three-month bills at a discount rate of 0.07 percent, down from 0.075 percent last week. Another $28 billion in six-month bills was auctioned at a discount rate of 0.110 percent, unchanged from last week. The three-month rate was the lowest since they averaged 0.065 percent on Jan. 7.