The fire at Ronald Ross Block of SSKM Hospital on Monday started from a cell phone tower on the sixth floor terrace, according to preliminary findings of a team comprising representatives of police, fire department and the hospital.

Team members said they were waiting for forensic reports to conclude how the tower - technically called micro-BTS and which serves at least four service providers - caught fire.

The blaze damaged a library being built on a part of the sixth floor, stalled two surgeries and exposed the inadequacies of an emergency response system that endangered the lives of 71 patients even after evacuation.

"We are still in the dark about how the tower caught fire," a senior SSKM official said.

A micro-BTS (Base Transreceiver Station) - a small tower installed on the roof of a building - is an instrument that facilitates wireless communication between a mobile phone and its network.

Experts in communication technologies said such towers, fitted with high-intensity new-age transmitters, usually had a UPS unit and an air-conditioner. A short circuit in the installation can always result in a spark, which can cause a fire.

The access to the terrace on which the tower is installed is through a flight of stairs. The only door to the terrace usually remains locked and its keys are in the custody of PWD officials in the hospital.

"We have found that no one visited the terrace on Monday morning or Sunday," a senior officer said. "So, the possibility of a sabotage can be ruled out."

Hospital sources said the tower, which is being maintained by a Delhi-based private company, was set up in 2012, when the entire sixth floor was a terrace. The company pays Rs 12,675 to SSKM's Rogi Kalyan Samity (patient welfare committee) every month.

"A few months back, a representative of the company had turned up and handed over a cheque clearing the dues for six months," an official said. "That was around the time when the hospital had launched a crackdown on a clutch of advertising companies which had set up billboards on the premises but were not paying anything."

Two years back, the hospital authorities had decided to put the terrace to use by setting up a library.

"The library was to be inaugurated in January. The PWD snapped the electrical connection in September following theft of copper ducts. The library does not have any power connection now," a hospital official said.

A meeting attended by representatives of the police, fire department, PWD and the hospital decided that a fire audit would be performed in each department over the next few months.

Besides, two fire tenders, including one that can rotate 360 degrees in a small radius, will be stationed on the premises.