A gang of burglars broke into the Dharmatala branch of Central Bank of India late on Wednesday and spent over four hours trying to prise open the vault.

They failed and left before the crack of dawn but not before fiddling with a computer and scattering bank documents all over the office floor.

A police officer said this was the first known instance of thieves entering a bank, spending so long inside and leaving without anything.

An officer of Taltala police station said the gang had changed the alarm mode from night to day after entering the bank by breaking the a window grille.

The alarm is attached to the vault in the basement. While in night mode, it starts ringing if anyone tries to open the vault. Officials change the mode from day to night before the branch closes for the day at 5pm.

A preliminary probe revealed that more than three persons were involved in the break-in.

“They seem to have entered the bank between 12.30am and 1.30am while it was raining heavily and left before dawn. They scaled the boundary wall in the rear and found one of the windows open,” said an officer. “The way the burglars went about their job suggest they had some technical expertise.”

“They sneaked inside by breaking one of the window’s grilles. Soon after entering the basement they switched on the lights,” said the official.

As they could lay their hands on two of the four keys needed to open the vault, the members tried their best to open the vault.

“When the desperation got better of them they planned to break it but could not,” said the official.

But unlike the film where the gang planted detonators to explode it, they tried to break open the vault with some iron rods.

“It seems they took turns to open the vault… And while someone was trying to open the vault, the other gang members kept themselves busy by switching on the computers and scanning documents,” said an official.

What has baffled the cops that how the gang got hold of the vault’s two of the four keys.

“The cashier has two keys while another two is kept with a woman employee. It seems one of them had left the keys on the table and it’s sheer negligence on their part,” he pointed out.

As usual the head peon of the bank, Govinda Haldar, turned up at 7.30am and opened the main gate to let the sweeper in. But he was in for a shock when he found all the tube lights switched on, drawers open and the window grills broken. He called up senior officials and later a complaint was lodged with Taltala police station.

Manisha Sikdar, branch manager, said that an internal inquiry is on to ascertain any negligence on the part of bank employees. A.N Bhowmick, zonal security officer of the bank, said: “Luckily, they could not break the vault. We have intensified our security and two guards will be posted at night from tonight.”

A police team led by deputy commissioner (central) Damayanti Sen inspected the bank and asked the bank officials to install a closed-circuit camera.