Regulators probing whether banks manipulated Libor have been asking banks under investigation to sign tolling agreements that the banks won't challenge enforcement actions by citing the statute of limitations, the Wall Street Journal reports. The Justice Department recently asked banks to sign such agreements and the Commodity Futures Trading Commission had made similar requests earlier this year.

The statute of limitation in some federal laws is set at five years, and the alleged manipulation of Libor goes back to the 2008 financial crisis and earlier. This summer, Barclays admitted that employees manipulated benchmark rates from 2005 through 2008.

Banks may agree to sign tolling agreements rather than push the government to file charges sooner, since charges could make documents publicly available that would support private lawsuits against the banks.