“The danger to whistle-blowers lies in a situation where the S.E.C. voluntarily declines to bring an action and hands everything off to another agency and steps away,” said Aegis J. Frumento, a lawyer at Stern Tannenbaum & Bell, who represents the SunTrust whistle-blower. “That’s a gap in the law that was probably not contemplated by Congress when Dodd-Frank was passed.”

Of course, it is up to the S.E.C. to decide which cases it believes are most worthwhile to pursue. It may have concluded that the material provided by the SunTrust whistle-blower doesn’t rise to that level. The S.E.C. does not comment on matters involving specific whistle-blowers.

Nothing in the recent SunTrust filing specifically ties the whistle-blower complaint directly to the Justice Department inquiry. Michael McCoy, a spokesman for SunTrust, declined to comment on the investigation other than to say that it involved “legacy mortgage matters” that the bank was working to resolve. Mr. McCoy maintained that the Justice investigation was unrelated to the whistle-blower complaint, as it involved another agency.

But Mr. Frumento said he had discussed the whistle-blower’s case with officials at both the S.E.C. and Justice Department. “It is my understanding that the S.E.C. shared the whistle-blower material with the Department of Justice,” he said.

That should come as no surprise, Mr. Frumento added, given that sharing such information among various law enforcement agencies was the idea behind the Financial Fraud Enforcement Task Force. Set up in 2009 by the federal government, the task force investigates an array of misdeeds that led to the credit crisis.

“SunTrust’s description of the investigation — that it concerns the origination and underwriting of loans to Fannie and Freddie — is exactly what our case is about,” he said. “It is inconceivable that someone would investigate the bank’s origination and sale of mortgages to Fannie and Freddie and not use our material.”

Certainly SunTrust has been buying back many of the loans it sold to Fannie and Freddie in the years before the crisis. Buybacks of this kind often indicate that the loans did not meet underwriting requirements. In its recent financial filing, SunTrust said it had repurchased $2.7 billion in mortgages over the last three years. Most of those repurchases came from Fannie and Freddie.