Calcutta/Birbhum, April 7: The blast in West Midnapore's Pingla in which at least 12 persons died has blown the veil off a cottage industry that thrives on explosives and employs thousands, many of whom are minors and are poorly paid.

Senior officials in the state crime records bureau and their counterparts in the home department said the business of manufacturing and marketing crude bombs and explosives had been flourishing in pockets of West Midnapore, Burdwan, Murshidabad and Birbhum since 2007, when the Nandigram anti-land acquisition movement was gathering steam.

"Under Trinamul rule, factories producing explosives have sprung up across the state. Only one industry is thriving in this state now and that is the explosives industry," CPM state secretary Surjya Kanta Mishra said after the Pingla blast.

Mishra's allegation against the Trinamul regime may sound like an exaggeration because some central Bengal districts have had a history of violence using crude bombs and explosives. But there is little doubt that crude bombs and explosives have become readily available across the state in the past few years.

The industry got a boost when politics in Bengal started getting polarised in 2008, said a senior police officer. He added that almost all elections since then had witnessed an exponential increase in the use of explosives.

Such has been the increase in the availability and use of explosives in the state that the Central Forensic Science Laboratory (CFSL) in Calcutta has started a pilot study to map districts that account for the highest number of cases relating to bomb explosions.

"Primarily, we have found that Murshidabad accounts for the highest number of cases, followed by the two Midnapores and Birbhum. The rest of the districts collectively make up a little over 10 per cent of cases relating to bomb explosions," said an official of the CFSL in Calcutta. He said the Park Circus-based CFSL office received the highest number of cases relating to bomb explosions in the country.

"The majority of the samples are low-intensity explosives containing charcoal, gun powder and compounds of sulphur and lead. Fuel oil is also used by the makers, who mostly operate from homes," the official said.

Figures available with the state police reveal that between January 2012 and June 2014, Murshidabad recorded 600 incidents involving explosives. Burdwan had 357 such cases and Birbhum 264.

Across rural Bengal, crude bombs are primarily of two types - round ones that have jute fibres wrapped tightly around a paper packet containing different explosive materials, andcylindrical ones in which iron pipes are filled to capacity with explosives including gunpowder and splinters.

The round ones, commonly known as peto, are more common.

Explaining how a peto is made, a manufacturer in Birbhum said the process begins by pouring different ingredients such as gunpowder, iron pins, broken glass and stone chips into a paper packet. The packet is then wrapped tightly with a thick layer of jute fibres. He said people with slender fingers were best suited for the job.

A well-made crude bomb can kill at least two persons. Such a bomb costs between Rs 300 and Rs 600. Market sources said the prices of the ingredients had shot up and so had the cost of labour. Gunpowder, for instance, now sells for Rs 3,200 a kg against Rs 2,600 a kg a few years ago. Bomb-makers come in groups and charge between Rs 2,000 and Rs 2,500 each a day.

"We have agents at Aurangabad in Murshidabad who supply gunpowder, the main ingredient. Earlier, CPM and Trinamul activists used to buy bombs. Now, the BJP has joined them," he said.

With political parties emerging as the biggest customers for such explosives, there is no dearth of ready labour, many of whom are teenagers. Sources said many youths were turning to bomb-making because of the lack of labour-intensive heavy industries.

"There have been instances of groups being called from Murshidabad to other districts to make bombs," a police officer in Birbhum said.

Police sources said gunpowder was mostly sourced from pockets of Aurangabad, Suti, Jangipur and Beldanga in Murshidabad. In Burdwan, gunpowder is sold in Ketugram, Katwa and Raniganj areas. Market operators in Birbhum claimed gunpowder was available in Nanoor, Parui, Labhpur and Muraroi in the district.

"Big-time dealers in explosives get explosives-manufacturing materials from Kishanganj and Munger in Bihar," he said.

Last October, the officer in charge of Parui police station, Prosenjit Dutta, was injured after crude bombs were hurled at him in Choumandalpur village when he went there in search of crude bombs.