By Mark Hosenball

(Reuters) - The U.S. attorney's office in Manhattan has opened an investigation of the cyber heist of $81 million from Bangladesh Bank's account at the Federal Reserve Bank of New York, a law enforcement source said.

Preet Bharara, the U.S. attorney for the Southern District of New York, is investigating the February crime, in which criminals used the SWIFT fund-transfer network to steal money from Bangladesh's central bank.

Bharara's office declined to comment.

The source spoke on condition of anonymity because he is not authorized to speak publicly.

The investigation by the federal prosecutor comes as the FBI and other government agencies are seeking to guard against other cyber thefts.

The Federal Reserve and other financial regulators last week told banks to review cyber-security protections against fraudulent money transfers in the wake of the Bangladesh Bank heist. The FBI last month privately urged banks to look for signs of attempted cyber thefts.

Last week, an FBI official speaking in Washington said the agency is investigating "a number of different tentacles" but does not yet know who committed the Bangladesh crime. The Bangladesh police and other law enforcement agencies also are investigating the largest known cyber heist from a bank.

A U.S. congressional committee has launched a probe into the New York Fed's handling of the heist.

Thieves in early February hacked into the Bangladesh bank's interface with the SWIFT network and peppered the New York Fed with fraudulent payment instructions. The New York Fed transferred $81 million held by Bangladesh Bank to accounts in the Philippines, where it went missing.

(Additional reporting by David Ingram in Washington)