A closed-circuit television camera recorded footage of two armed guards collapsing and choking to death in the vault of a private bank minutes after switching on a generator that filled the ventilation-less room with noxious fumes.

Police prised open the locked shutters to ING Vysya Bank’s vault on BT Road on Monday morning to find guards Deboprosad Baidya, 50, and Basudeb Ghosh, 38, sprawled on the floor a few metres apart in the antechamber to the strongroom that holds a row of currency chests.

The petrol generator was running despite power having returned around 4am after an hour of loadshedding. The 5,000sq ft vault, a cavernous layout of chambers with multiple doors and a rectangular strongroom, was swimming in smoke. The CCTV, a silent witness to the tragedy, was on.

“The CCTV footage shows Ghosh gasping for breath and trying to enter the server room before collapsing on the floor at 3.20am. Baidya is seen lifting himself to walk towards the storeroom, only to slump again. He tries to make a call from his cellphone but can’t,” an investigator said.

The generator apparently emitted a mixture of gases like carbon monoxide, carbon dioxide and hydrogen sulphide, according to experts.

“The lack of a ventilator meant the gas quickly filled the room, aided by the air-conditioner. One of the guards apparently tried to switch off the generator before losing consciousness. The duo possibly died of carbon monoxide poisoning, but we can confirm that only after conducting a set of tests,” said Dhurjyoti Prasad Sengupta, the director of the state forensic laboratory.

The combination of gases that filled the vault was so strong that several police and bank officials were taken ill after inhaling the fumes during the investigation. Five police officers and two bank employees were hospitalised in the afternoon with symptoms of air poisoning like dizziness, nausea and irritation in the eyes.

ING Vysya Bank had started using the Cossipore vault barely a week ago and Monday’s power cut was probably the first in that facility, the police said.

The two victims, employees of Lokenath Security Agency, were on duty at the vault since 7pm on Sunday. “Our currency manager visited the facility around 9am and found the shutters down. He repeatedly banged on the iron shutters and called up the landline number, but received no response. The police were informed around 10am. They had to get past a three-layered security system comprising shutters locked from inside, a wooden door and a collapsible gate,” said Proloy Mukherjee, the cluster head (retail) of ING Vysya Bank.

Ghosh was found first, lying face down over his rifle. Baidya was on his back, his right hand clutching his cellphone and left hand holding the leg of a chair that he seemed to have dragged down in his struggle to breathe. The portable generator was revving in the storeroom to the left of the entrance. To the right hung a mosquito net, suggesting that the guards were inside it before the power cut forced them out.

Baidya, an ex-army jawan, was from Hasnabad while Basu was a resident of Gaighata, in North 24-Parganas.

Joint commissioner (crime) Damayanti Sen said the police would file a suo motu case against the bank for negligence.