It is the largest case of its kind in Costa Rica's history.
"We cannot say for sure that this is fraud or that someone stole this cash," the bank's interim manager Jaime Murillo told a press conference on Tuesday.
"We are not at that point yet. It is under investigation."
He said the missing amount is equivalent to 0.06% of the bank's assets.
What do we know about the suspected heist?
Bank staff noticed the money was missing on October 3 but Murillo said the incident must have taken place in August or earlier.
The bank suspended five workers after the issue came to light and filed complaints against them for embezzlement and a breach of duties.
They include two area supervisors, a technician, a guard and an accountant — all of whom worked worked in the area of the bank that handles currency. No arrests have been made.
Clients of the state-owned bank are not at risk, Murillo added.
