The U.S. Justice Department has subpoenaed documents from Clayton Holdings, a mortgage due-diligence firm, as the agency ratchets up its investigation into the firm’s work with residential mortgage-backed securities. Bloomberg News says:

Information sought includes due diligence reports, internal communications related to reviews of pools of loans and correspondence with clients, according to a copy of the subpoena filed as an exhibit in federal court.

The work of Clayton in the years before the subprime mortgage crisis has been used by state and federal agencies in past cases. The Shelton, Connecticut-based firm so far has declined to comply with the subpoena, according to a Justice Department court filing on Aug. 27.