Quoting sources familiar with the matter, the Journal said FBI agents investigating the case "have found evidence pointing to at least one bank employee acting as an accomplice."
But it added that "a handful of others" may have also aided the hackers in breaking into the computers of Bangladesh Bank.
The spectacular cyber-theft has embarrassed the government in Dhaka, triggered outrage in the impoverished country and raised alarm over the security of the global infrastructure linking central banks.
But the New York branch of the US central bank blocked most of the bogus transfer orders it received, preventing the theft from skyrocketing to USD 1 billion.
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The involvement of the New York Fed has brought the FBI into the case, but the Fed is not being viewed as blameworthy.
Separately, the global financial transfers network SWIFT today rejected reported accusations by Bangladesh police and bank officials that it was to blame for low security protections.
"As a SWIFT user like any other, Bangladesh Bank is responsible for the security of its own systems interfacing with the SWIFT network and their related environment -- starting with basic password protection practices -- in much the same way as they are responsible for their other internal security considerations."
SWIFT said its officials would be meeting with representatives of Bangladesh Bank and the New York Fed in Basel on May 10.