In the heist attempted on Saturday-Sunday night when the bank was closed, the thieves broke open 17 lockers and decamped with Rs 28 lakh in cash and jewellery valued at over Rs 70 lakh.
Police said the incident came to light on Monday morning when west Delhi's Mundaka-located Cooperative Bank was opened by its manager Anup Singh, the complainant.
After filing a case, the bank officials are busy estimating the exact value of the stolen valuables.
"Prima facie, the theft was carried out on the intervening night of Saturday and Sunday. A 10-feet-long tunnel up to the bank was dug up from near a wall adjacent to a vacant plot to enter the bank," Additional Deputy Commissioner of Police Amit Sharma said.
He said the thieves cut the wires of security alarm and Digital Video Recorder (DVR) connected to the CCTV camera.
The thieves stayed in the bank for over five hours and were apparently familiar with the location of valuables and cash kept in different lockers in the bank, Sharma said.
The officer said it was a pre-planned crime and that the thieves possibly had done several round of recce of the bank and its topography before attempting the heist.
The police also did not rule out the involvement of bank employees. Six teams have been formed for the probe.
"Mobile phone locations active near the spot are also under scanner," Sharma added.