J.P. Morgan Chase & Co. is returning to a nearly deserted part of the credit markets with a sale of $616 million of securities tied to mortgages with no backing by the U.S. government.

J.P. Morgan's offering is supported by 752 jumbo mortgage loans, most of which of which were originated by J.P. Morgan and First Republic Bank, said Fitch Ratings on Wednesday, giving most of deal the highest credit rating, triple-A. The loans were made to borrowers with high credit scores and with about 35% of their own money used for a...