The U.S. Securities and Exchange Commission is being helped by a whistle-blower in an investigation of Deutsche Bank AG’s post-crisis mortgage-trading business, according to people with knowledge of the situation.

The SEC received a whistle-blower complaint alleging that the bank inflated the value of mortgage bonds on its books and masked losses around 2013, said the people, who asked not to be identified because the matter is confidential. The SEC is probing how the bank valued government-backed mortgage bonds known as Agency pass-through securities that it amassed after the 2007 U.S. housing crisis.

Investigators are looking at positions overseen by Troy Dixon, who ran the bank’s trading for U.S. government-backed mortgage bonds after the crisis until he left the bank in October 2013, people with knowledge of the matter said last month. SEC investigators want to know whether the bank delayed recording losses over an extended period of time on pass-through securities with high coupons, the people said.

Judith Burns, an SEC spokeswoman, declined to comment. Patrick Clifford, a spokesman for Dixon, declined to comment.

Deutsche Bank’s losses for Agency pass-through securities surged to 394 million euros for the whole of 2013, or more than $500 million at the time, according to trading documents seen by Bloomberg.

A spokeswoman for Deutsche Bank declined to comment and instead referred to a statement that the bank provided Bloomberg last month that said it was cooperating with the SEC’s investigation. In that statement, she said losses tied to the mortgage bonds have been recognized by the bank, without saying when the losses were reported.

Whistle-blowers with the SEC can receive rewards for their information, ranging between 10 percent and 30 percent of the money the agency collects for sanctions over $1 million. Informants "can be among the most powerful weapons in the law enforcement arsenal" of the agency, according to the website for the SEC’s whistle-blower office.

Deutsche Bank’s Chief Executive Officer John Cryan last year announced his intention for the bank to completely exit the business of trading U.S. Agency mortgage-backed securities.

The German lender has been grappling with a number of legal and law enforcement challenges around the world that Cryan has vowed to resolve. The bank last year hired as its general counsel for the Americas a former official at the U.S. Department of Justice and attorney for former President Bill Clinton.