Investors led by BlackRock and Pacific Investment Management Co are zeroing in on a new target in their bid to recoup losses tied to the US housing bust: the so-called trust banks that oversee payments and enforce terms on more than $2 trillion in residential mortgage securities.

A group led by the giant investment firms on Wednesday sued units of Deutsche Bank and US Bancorp in New York Supreme Court for their roles as trustees overseeing the bonds on behalf of investors who bought the securities.