The Securities and Exchange Commission (SEC) is reportedly investigating lenders’ procedures for packaging home mortgages into securities bonds for sale to investors.

Reuters, citing two sources familiar with the probe, says the SEC sent subpoenas last week to Bank of America, Citigroup, JPMorgan Chase, Goldman Sachs, and Wells Fargo.

The news agency says the subpoenas focus on the earliest stage of the mortgage securitization process, in particular, the role of master servicers who manage the selection and maintenance of the home loan pools that go into mortgage-backed bonds, and whether or not the loans were properly transferred to the trusts that issued the securities.

Sources also told Reuters that the SEC is seeking information about the role banks had in mortgage securitization, and the role trustees that issued the mortgage-backed securities (MBS) had in monitoring the performance of the underlying loans.

Questions about what entities had the legal right to foreclose on mortgages packaged as securities, as well as whether or not transfers of ownership were properly recorded when the loans were sold to investors, emerged when the recent robo-signing scandal surfaced and scrutiny of servicers’ documentation procedures intensified.