Bangladesh police are reviewing a nearly forgotten 2013 cyber heist at the nation’s largest commercial bank for connections to February's $81 million heist at the country's central bank, a senior law enforcement official said on Wednesday.

The unsolved theft of $250,000 at Sonali Bank involved fraudulent transfer requests sent over the SWIFT international payments network. It is not widely known outside of Bangladesh, and in fact was treated as a cold case until Bangladesh police revived the investigation after thieves in February also used the SWIFT network to steal $81 million from Bangladesh Bank.

Sonali Bank said it had informed SWIFT about the 2013 heist at the time and also unsuccessfully tried to recover the money from the recipients in Turkey, said one bank source.

Thieves in the 2013 robbery used tactics similar to those used by the yet-to-be-identified criminals in the Bangladesh Bank heist--using the SWIFT money-transfer system to divert bank funds, said a senior bank official. Authorities are now reviewing the case to see if there are any links that can help them track down the criminals behind the Bangladesh Bank heist.

At Sonali Bank, hackers installed key-logger software on a computer to gain passwords to other systems, then sent fraudulent transfer requests over SWIFT, said the senior bank official who is part of its IT operations.

Police arrested two employees who had responsibility for initiating and approving money transfer instructions, but they were later freed without being charged.

Sonali Bank Managing Director Pradip Kumar Dutta told Reuters that the attackers remain at large and no money has been recovered.

 “We could not find out what happened," the official said.

The Sonali Bank cyber heist is the fourth documented case involving fraudulent SWIFT messages and the earliest known case to surface. It is not known whether any of the robberies, including the two attacks on Bangladesh banks, are related.

The two other cases that have come to light are a $12 million theft from Banco del Austro in Ecuador in January and an attack on Vietnam's Tien Phong Bank in December that was not successful. The Sonali Bank theft was reported by Bangladesh media at the time, but has faded from public memory. Police said they only recently became aware of similarities with the central bank heist.

"This is an interesting issue that we've come to know," said the senior police official, who declined to be identified further. "We'll have to look into it."

News of these attacks has tested faith in the security of SWIFT, a key conduit for global financial transactions that is used by more than 11,000 banks and other institutions. Regulators and banks have already implemented reviews of SWIFT security measures to determine whether other banks could be vulnerable to similar attacks. SWIFT spokeswoman Natasha de Teran declined to comment on the Sonali case.

"We are actively looking into other possible instances of such fraud, but we will not comment on individual entities," she said.

Bangladesh's Anti Corruption Commission, which investigated the Sonali case, did not have an immediate comment.