Thieves dug a 100-foot-long (30-meter-long) ventilated and lit tunnel from a neighboring building into an Argentine bank and emptied the contents of up to 140 safety deposit boxes, officials said Monday.

Authorities said three thieves entered a Banco Provincia branch in the Buenos Aires district of Belgrano on New Year's Eve when it was closed and spent the weekend opening and emptying between 130 and 140 of the branch's 1,408 boxes. The robbery wasn't discovered until the bank opened Monday.

Bank executives didn't say how much the thieves got away with because clients are not obliged to tell authorities what was in their safety deposit boxes.

The thieves rented a building next to the bank in June and spent months digging the tunnel, which emerged in an area where safety deposit boxes are located, prosecutor Martin Niklison said.

The tunnel had lights, ventilation and reinforcement.

"It was a really impressive job," Niklison said, adding that officials had not ruled out that the thieves had an accomplice on the bank's staff.

Anti-seismic alarms sounded various times but police couldn't get into the bank to check out their cause, the prosecutor said.

Hundreds of bank clients gathered in front of the state bank Monday to demand their savings and blocked a nearby street to protest the slow pace at which authorities were confirming whose boxes had been robbed.

"The message we want to give (clients) is that we are going to protect their interests after this incident," Banco Provincia Vice President Gustavo Marangoni told channel C5N.

Many Argentines began putting their savings in bank safety deposit boxes instead of bank accounts following the country's 2001 financial crisis, when many depositors lost savings.