WASHINGTON — Long-term U.S. mortgage rates fell this week to the lowest level since May 2013, driven down by financial tumult in Europe.

Mortgage giant Freddie Mac said the average 30-year fixed rate mortgage fell to 3.41 percent from 3.48 percent a week ago.

A year ago, the 30-year rate stood at 4.04 percent.

The 15-year mortgage rate dropped to 2.74 percent, down from 2.78 percent last week and 3.20 percent a year ago.

After Britain’s recent vote to leave the European Union, worried investors fled to the safety of U.S. Treasury bonds. Long-term mortgage rates tend to track the yield on 10-year Treasury notes, which fell to 1.37 percent Wednesday from 1.75 percent before the Brexit vote.

The 30-year fixed rate is now close to its all-time low of 3.31 percent in November 2012.

To calculate average mortgage rates, Freddie Mac surveys lenders across the country at the beginning of each week.

The average doesn’t include extra fees, known as points, which most borrowers must pay to get the lowest rates. One point equals 1 percent of the loan amount.

The average fee for a 30-year mortgage remained at 0.5 point this week. The fee for a 15-year loan was unchanged at 0.4 point.