Walter Investment Management Corp. is buying San Diego-based Security One Lending in a $31 million deal, bolstering its portfolio of reverse mortgages.

Tampa-based Walter Investment, which specializes in handling subprime and other “credit-challenged” mortgages, initially made a big foray into reverse mortgages with the $120 million acquisition of a Texas mortgage company in September.

Its new target, Security 1, originated nearly 5,000 reverse mortgage loans last year with an total principal balance of more than $700 million.