Banks bound by cooperation agreements in an interest-rate rigging probe are providing a windfall of information to U.S. prosecutors investigating possible currency manipulation, according to a Justice Department official and a person familiar with the matter.

"The cooperation that we have been able to secure as part of our agreements in the Libor investigation has been very helpful to us in terms of holding banks' feet to the fire," Mythili Raman, the acting head of the Justice Department's criminal division, said in an interview today. Raman, who declined to name the banks or comment further on the probe, said banks are "providing us with information about any new or ongoing conduct—even old conduct—that might be coming to light now."

The accords have compelled some lenders to conduct internal examinations of their foreign-exchange (FX) businesses and share findings with the Justice Department, speeding up the government's criminal probe into the $5.3 trillion-a-day market, according to a person with knowledge of the investigation.