Robbers tunneled from a basement into the sewer before traveling up and into a bank vault. Authorities reportedly found leftover equipment inside the sewer.
Robbers raided dozens of safe deposit boxes from a bank in Belgium at the weekend after tunneling in, prosecutors said.
The thieves tunneled up into the vault of an Antwerp branch of BNP Paribas via a nearby sewer system.
At 1:34 p.m. (12:34 UTC) the bank's alarm was triggered despite the vault door remaining locked. By the time authorities made it into the vault, thieves had opened and robbed 20 to 30 deposit boxes and fled into the tunnel.
A search of the sewer in Antwerp's famed diamond district found a second four-meter (13-foot) tunnel that led into the basement of an apartment building several hundred meters away from the bank. The robbers presumably descended into the sewer system from the basement, traveling through the sewer before ascending into the bank vault.
Police have not said what was stolen, but bank customers were queuing at the branch on Monday to find out if they were victims of the theft.
"We are investigating the size of the robbery and we cannot provide further information," the prosecutor's office said in a statement.
The police have no major suspects.
Apartment residents questioned
Local paper Het Laatste Nieuws reported the basement of the building had been vacant for some time, citing residents. Some residents of the building were reportedly interviewed by police but released without charge.
Equipment, including a mattress, was found in the sewer.
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Municipal authorities told Belgian radio the raid was very dangerous for the thieves, saying they risked drowning from fast-moving rain water or asphyxiation from noxious gases.
Sewer pipes in the city can be as big as 1.3 meters high and as wide as 80 centimeters. But they can also be considerably smaller and often have obstacles inside.