Mortgage rates fell to the lowest level in decades for the eighth time in nine weeks, a sign that investors are concerned about the weak economy.

The average rate for 30-year fixed loans this week was 4.42 percent, down from 4.44 percent last week, mortgage buyer Freddie Mac said Thursday. That's the lowest since Freddie Mac began tracking rates in 1971.

The average rate on 15-year fixed loans dropped to 3.9 percent, down from 3.92 percent last week and the lowest on records dating back to 1991.

Rates have fallen since spring as investors sought the safety of Treasury bonds, lowering their yield. Mortgage rates tend to track those yields.

Falling rates have pushed refinancing of home loans to the highest level since May 2009. But it's still lower than during the first three months of that year, when rates first fell to around 5 percent.

Low mortgage rates, however, have failed to spark home sales. They remain hobbled by the weak economy and tight credit standards.

Rates have fallen since spring as investors sought the safety of Treasury bonds, lowering their yield. Mortgage rates tend to track those yields.

To calculate the national average, Freddie Mac collects mortgage rates on Monday through Wednesday of each week from lenders around the country. Rates often fluctuate significantly, even within a given day.

Average rates on five-year adjustable-rate mortgages were unchanged at 3.56 percent. Rates on one-year adjustable-rate mortgages also were unchanged at an average of 3.53 percent.

The rates do not include add-on fees known as points. One point is equal to 1 percent of the total loan amount. The nationwide fee for loans in Freddie Mac's survey averaged 0.7 a point for 30-year and 1-year mortgages. They averaged 0.6 of a point for 15-year and 5-year mortgages.