Fannie Mae, the largest source of money for U.S. mortgages, sold $750 million of securities in its second offering of notes tied to the risk of homeowner defaults.

The sale included $375 million of senior-ranked bonds with grades of Baa2 from Moody’s Investors Service and one step lower at BBB- from Fitch Ratings, the Washington-based company said Wednesday in an emailed statement. The rest of the debt isn’t rated. The junior notes would lose principal first amid defaults on a pool of about $29 billion of loans, Fitch said in a pre-sale report on the deal.