A late night explosion on Tuesday blew up a yet-to-be-opened restaurant and caused extensive damage to adjoining shops in one of Ranchi's commercial hubs, with forensic sleuths suspecting accumulation of methane gas in a clogged drain to be the culprit.

The blast took place around 10.50pm at Dessert Delite, a refreshment joint in Rangrez Gali of Upper Bazar, which is slated for inauguration on Saturday.

Referring to preliminary forensic findings, Kotwali OC Vijay Kumar Singh said this was the first time he had heard and seen the danger a clogged drain could pose.

"When we heard of the explosion at Rangrez Gali, we were on our toes, suspecting criminal/rebel handiwork, because the shop is barely half a kilometre from the thana. But, people from the Forensic Science Laboratory did not find traces of explosives. The team has hinted that methane gas, which accumulated in the drain, triggered the explosion," Singh said.

On how and why the bizarre "methane bomb" might have formed, the officer said that the owner, Bhavin Rathore, had covered up the drain running along the entrance of his eatery to make it more customer-friendly. "Now, waste from everywhere gathered in the drain, which had no opening or ventilation. It rotted and formed methane," the policeman quoted forensic experts as saying.

But, even a combustible gas would require a spark to cause an explosion, the magnitude of which is still questionable as it ripped out the shop's shutter and collapsible gate, wrecked the interior completely and effected various degrees of damage within a 50-metre radius.

So, where was the spark? "Well, after methane was produced in huge quantity, it found an access into the shop through a drain pipe. But, the shop had no exhaust fan running. Methane combined with oxygen to form an explosive mixture of gases, which got the spark from the refrigerator and went off," the OC said.

The two members of the forensic team refused to speak to the media.

State police spokesperson S.N. Pradhan echoed his junior in a WhatsApp reply to The Telegraph 's queries. "Most likely cause... methane gas in drain," the message read.

DIG Arun Kumar Singh, who had inspected the site and pressed a sniffer dog into service, said they scoured through the debris of the shop for the whole night, but nothing suggested sabotage.

"I still don't believe (that it is a gas explosion)," said Rathore, but parried questions on why he had blocked the drain.

FJCCI president Ratan Modi said most shops remained closed in Upper Bazar area on Wednesday in the wake of the explosion. "We are still trying to digest the news. I have never heard of a clogged drain going up like that," Modi said.