Units of bond insurers Ambac Financial Group Inc, MBIA Inc, Syncora Holdings Ltd and Assured Guaranty Ltd have also been subpoenaed by the state's attorney general for information related to claims they paid out on mortgage-related products, according to this person.

The person was not authorized to speak publicly about the probe.

The office of New York Attorney General Eric Schneiderman is also seeking information about litigation between the bond insurers and banks that packaged mortgage securities, this person said.

Representatives from Ambac, Deutsche Bank, JPMorgan and UBS declined to comment. Calls to RBS, Ambac and Syncora were not immediately returned.

MBIA spokesman Kevin Brown confirmed the company had received a subpoena, which he said seeks information related to various complaints MBIA has filed against various financial institutions over the quality of mortgage securities MBIA insured.

Ashweeta Durani, a spokeswoman for Assured Guaranty, said in a statement the company supports the investigation, but would not confirm whether it had received a subpoena.

Reuters reported last week that Schneiderman already has asked Bank of America Corp, Goldman Sachs Group Inc and Morgan Stanley for information on their mortgage securitization practices.

The probe comes amid a flurry of regulatory activity into how financial institutions handled home loans and assembled them into pools sold to investors.

Earlier this month, the Department of Justice sued Deutsche Bank for more than $1 billion for allegedly misleading the Federal Housing Administration about the quality of its mortgages in an effort to obtain insurance for them and later sell them. The government contends it had to pay claims on mortgages that did not meet its standards.

Banks' foreclosure practices also are under investigation by all 50 U.S. state attorneys general, as well as the Justice Department and the Securities and Exchange Commission. The SEC is also investigating how banks bundled and sold toxic mortgages in collateralized debt obligation deals.