Fifteen-year mortgages dropped to a historic low last week while the 30-year rate came close to hitting bottom on the heels of the nation’s top bank pumping cash into a troubled real estate market.

“The Federal Reserve purchased more than $1 trillion worth of mortgage-backed securities from Fannie Mae and Freddie Mac and that’s driving down mortgage rates,” said Karl Case, an economist at Wellesley College. “The Fed wants to save this housing market at all costs.”