ING said it was seeking more information on what happened
Belgian police are hunting for a man who stole about 60,000 euros (£54,000) from a bank in the European Parliament, after brandishing a pistol at staff.
It was not yet known if the pistol was real or fake, parliament spokesman Jaume Duch Guillot told the BBC.
The man demanded money at an ING bank branch on Thursday after evading checks by the parliament's security personnel. The robbery lasted only a few minutes.
The security service is now studying CCTV footage for clues about the man.
On Friday, Belgian police said he had taken about 60,000 euros in cash.
After the robber fled, staff alerted a nearby security patrol.
Some MEPs and their staff were locked in their offices during the search of the building. Their bags were then checked as they left.
Mr Guillot said that if the robber's weapon had been real, it "would pose questions" about security at the EU complex, "notably to know what badge he used to get in".
However, he noted that the sprawling complex, which occupies several blocks in central Brussels, was visited by 15,000 people a day, making it difficult to maintain foolproof security at all times.
"One hundred percent security never exists, but this is a public and a very open parliament and we prefer still to be a very open parliament," he said.