Bangladesh's central bank is unlikely to extend the contract of US cyber security firm FireEye to investigate the electronic theft of $81 million of its money, sources at the bank say, citing high costs as one of the factors.

US cyber security firm FireEye, which is charging Bangladesh Bank $400 an hour to probe the $101 million heist, is hunting for a malware that it believes is still hiding in the bank's network.

FireEye Inc's Mandiant forensics division is helping to investigate a cyber heist at Bangladesh's central bank last month that netted more than $80 million, people familiar with the matter tell Reuters.