The blast in Duttapukur has exposed chinks in the policing of illegal fireworks units that have continued to thrive, defying chief minister Mamata Banerjee’s directive to crack down on such set-ups following a similar explosion in East Midnapore’s Egra in May.

Senior officers at the state police headquarters said the firecracker unit at Mochpole village couldn’t have thrived without the knowledge of law enforcers at the local Duttapukur police station.

“In the aftermath of the blast in Egra, police stations were instructed to carry out a special drive against illegal cracker units following orders from the chief minister,” said a senior police officer at the headquarters in Calcutta. “Among several places where such units were raided was Duttapukur in North 24-Parganas where a huge quantity of explosives was recovered. This blast comes within three months of that recovery.”

Mamata had said the Egra blast could have been averted if “the intelligence had worked properly.”

Senior officers said with Duttapukur, there was no shortage of intelligence but officers in the Barasat Sadar subdivision chose to gloss over the inputs.

A very basic observation of the nature of Sunday’s blast has led a section of forensic experts to believe there was a huge stockpile of organic chemicals in the form of ammonium nitrate and fuel oil inside the house.

“Given the nature of explosion where a concrete pillar has been twisted, it appears there was a ‘detonation’ — a phenomenon where a high volume of organic chemicals goes off in a small place creating huge pressure,” said P.K. Ghosh, former head of the Central Forensic Science Laboratory in Calcutta.

“The other is deflagration where a series of small intensity blasts occurs following the transfer of heat, something that happens in case of stocked firecrackers bursting.”

The police have found the existence of a makeshift laboratory under a plastic shed some 300 metres away from the blast site. Located near a bamboo grove, it had a series of glass beakers, gloves and huge plastic containers with chemicals stored.

Workers from Murshidabad were employed at the unit which flourished despite protests from a section of local residents, senior police officers have learnt. A team from the Duttapukur police station had visited the site on Saturday evening, they said.

“The local police station has to be aware of the illegal fireworks unit because of the presence of a group of migrant workers in a small pocket where a huge stock of explosives is kept in a house for commercial purposes,” the senior police officer said. “The police chose to gloss over it.”