Investigators examining the theft of $81 million fromBangladesh's central bank have uncovered evidence of three hackinggroups — including two nation states — inside the bank's networkbut say it was the third, unidentified group that pulled off theheist, according to two people briefed on the progress of thebank's internal investigation.

FireEye Inc., the company hired by the bank to conduct theforensics investigation, identified digital fingerprints of hackinggroups from Pakistan and North Korea, the two people said. Ithasn't found enough data to determine whether the third group, theactual culprit, was a criminal network or the agent of anothernation.

The twists and turns add to the mystery of who pulled off one ofthe largest cyberheists in history. The hackers, pairing theft withhavoc within the global financial system, used the Swift interbankmessaging system to move cash into fake accounts in the Philippinesbut were discovered before they could complete an attemptedtransfer totaling $951 million.