Sean Kanuck, who was the most senior official in charge of cyber security at the Office of the Director of National Intelligence for five years until mid-May, told Reuters that there had been no official determination on who committed the cyber heist, one of the biggest ever.
"They may never be able to make one," Kanuck said on the sidelines of the annual Shangri-La Dialogue, Asia's premier security forum, held at the weekend in Singapore.
He said he had some knowledge of the case but was not directly involved in the probe.
Investigations into the heist are being coordinated by the US Federal Bureau of Investigation. The authorities in Bangladesh, the Philippines and some other countries are also carrying out inquiries.
The hackers stole money from Bangladesh Bank's account at the New York Federal Reserve. One fraudulent transfer to a Sri Lankan entity was reversed, but four transfers for a combined $81 million went to the Philippines and wound up being laundered through casinos and casino agents there.
Most of the money remains missing.
Kanuck said that he believed either an extremely sophisticated criminal group or a rogue nation carried out the theft.
BAE Systems has said malware used to erase the tracks of hackers in the Bangladesh Bank heist was similar to code used to attack Sony Corp in 2014, a strike blamed by the FBI on North Korea.
"We have actually seen criminal enterprises that were able to bring together a range of capabilities, ranging from insider access to credentials, going through to people who were willing to go physically remove money from ATMs," said Kanuck.