Twelve hours and several theories later, the mystery blast beneath a tree on Shakespeare Sarani on Wednesday remained a mystery.

“We are yet to know what had triggered the underground blast. The Calcutta Municipal Corporation and forensic experts have been asked to send their reports,” said Jawed Shamim, the deputy commissioner of police (detective department).

Minutes after the 9.45am blast, several theories started doing the rounds, but none proved to be conclusive.

Police first issued a statement, saying a transformer had exploded. But there was no transformer on the spot.

A source blamed the goof-up on a senior officer of the traffic department. “The officer rushed to the spot immediately after the blast and saw a distorted iron vat. He mistook it for an exploded transformer and briefed Lalbazar accordingly,” said the source.

Next to come to the spot, an hour later, was police commissioner Gautam Mohan Chakrabarti, who said “an electrical short-circuit” might have led to the blast.

But CESC men dug up the area around 3pm and found the cable intact. “We dug up the cable and found it intact. There was also no feeder fault,” said Dilip Sen, the executive director (commercial and mains) of CESC. “Besides, had there been a cable fault, supply would have been disrupted. No power cut was reported from the area,” said an official of the power utility.

As soon as the second theory was scotched, emerged a third. The police started saying a leak from an underground gas pipeline might have caused the blast. “Leaked gas creates enormous pressure that could trigger an explosion,” an officer tried to explain.

The civic body added weight to the conjecture, with chief engineer (drainage and sewerage) Tushar Ghosh saying: “A leak of propane from a pipeline of the Greater Calcutta Gas Supply Corporation had triggered the blast.”

But the possibility had to be ruled out after men from the gas company dug up the area and found the junction in the pipelines okay. “There was nothing wrong with the pressure of the gas,” admitted a police officer. “Had there been a leak, we could have detected it by smell.”

Nanda Samui, the general manager of Greater Calcutta Gas Supply Corporation, said another round of tests would be carried out on Wednesday.

The police then came up with a fourth idea, identifying methane accumulated in sewer lines as the villain. “But to substantiate the theory, we have to wait for the civic body to examine the sewerage system,” said deputy commissioner Shamim.

Civic chief engineer Ghosh, however, said a methane-induced blast would have blown off the manhole cover. “This hasn’t happened.”

The fifth conjecture — floated by gossip in the area — of explosives planted under the tree was rejected by the police. “There were no scorch marks at the site. And none of the injured sustained burns,” said Shamim.