WASHINGTON (AP) — Average long-term U.S. mortgage rates were mixed this week, with the key 30-year rate slipping back under the 4 percent mark.

Mortgage buyer Freddie Mac says the average rate on a 30-year fixed-rate mortgage declined to 3.97 percent from 4.01 percent a week earlier. That rate has risen from its 3.73 percent average a year ago. Still, borrowing costs are well below the historic average of 6 percent for a 30-year loan.

The average rate on 15-year fixed-rate mortgages this week rose to 3.26 percent from 3.24 percent.

Rates rose slightly in the final week of 2015 in the wake of the Federal Reserve's decision in December to raise a key short-term interest rate.