WASHINGTON (AP) — The average U.S. mortgage rate fell below a key threshold of 4 percent this week, its lowest level in five months.

Mortgage buyer Freddie Mac said Thursday that the average interest rate on 30-year fixed-rate home loans declined to 3.97 percent this week from 4.08 percent last week. Interest rates on mortgages began to rise after President Donald Trump won the November election. But they've started falling as the fate of tax reform and other policies has become uncertain.

The 30-year rate stood at 3.59 percent a year ago and averaged 3.65 percent in 2016, the lowest level in records dating to 1971. Lower rates make it easier for homebuyers to afford their monthly mortgage payments.

The rate on 15-year mortgages declined to 3.23 percent from 3.34 percent last week.

The recent drop also illustrates the range of factors that affect mortgage rates. The average 30-year rate has declined steadily in recent weeks — it was 4.23 percent a month ago — even as the Federal Reserve has lifted the short-term rate it controls three times in the past 15 months.

And Fed policymakers have signaled more hikes are likely to come this year as long as the economy keeps growing.

Mortgage rates, however, more closely track the yield on the 10-year Treasury note, rather than the Fed's decisions. That yield rose after the election in anticipation of faster growth and greater inflation under President Trump.

Yet as investors have downgraded their expectations for tax cuts and infrastructure spending, the yield on the 10-year has fallen. That has led mortgage rates lower as well.