An explosion early on Tuesday inside a club in Beleghata destroyed a portion of the wall and the asbestos roof of a small room on the rooftop.

Bricks from the damaged concrete wall flew and landed around 40ft away leaving people of the locality terrified. No one was injured, police said.

Several residents said they heard a deafening sound around 6.30am. Some said the sound could be heard almost half a kilometre away, close to Gandhi Bhawan.

“It was quite early in the morning… there were a few people on the road. I had stepped out for a cup of tea from a nearby stall when there was this sudden loud noise… we could see black smoke from a distance,” Tapas Malakar, a resident of the area, said. “It took us a while to understand the explosion had occurred inside the club building. Sometime later members of the bomb disposal squad arrived.”

The police have sealed the Beleghata Gandhi Math Friends Circle Club, a double-storey building with a small room on the rooftop.

A team of investigators from the city police’s detective department, along with officers of the dog squad and from Beleghata police station visited the club several times during the day, collecting samples and talking to some club members.

“Such an impact usually happens when some high explosive substance is stored in a small place. There has to be the presence of ammonium nitrate that got mixed with fuel oil,” P.K. Ghosh, a former senior scientist with the Central Forensic Science Laboratory in Calcutta, said. “Left untouched a mixture of ammonium nitrate and fuel oil will not explode. There must have been some trigger through some shock.”

There is presence of soot on the wall. “We will examine the place once again on Wednesday to get a clear understanding of the nature of explosion,” a member of the forensic team said.

Some club members said no explosive material had been stored inside the club building. They told cops that people on motorcycles with their faces covered had hurled bombs at the building.

A few members said gas cylinders had been kept on the second floor and that they had exploded.

It will take some time to piece together the information that has been emerging before arriving at a conclusion, senior police officers, speaking off the record, said.

“The most crucial will be the preliminary forensic report,” an officer overseeing the investigation said.

BJP leader Locket Chatterjee demanded the National Investigation Agency look into the blast.