The National Investigation Agency or NIA of India on Monday filed a chargesheet in a Kolkata court against 21 people, including four Bangladeshi nationals, more than six months after an explosion in a house in West Bengal's Burdwan district, reports NDTV.

A press statement issued by the NIA says, the chargesheet has been filed against the accused ‘for their involvement in the conspiracy of Jamaat-ul-Mujahideen, Bangladesh (JMB), a proscribed armed organisation in Bangladesh, to overthrow the existing democratic Government in Bangladesh through violent terrorist acts and replace it with a hard-line Sharia based Islamic rule, which came to light after the accidental blast.’

The NIA claims the armed outfit, JMB, has established its network in the states of West Bengal, Assam and Jharkhand. ‘Its activities in India primarily included recruitment, radicalisation and training of vulnerable youth in a systematic and organised manner,’ said the agency in its statement.

The NIA further alleges, ‘A network of terrorist training camps at selected madrasas and other hideouts was found to be in operation.’

The investigating agency has examined more than 250 witnesses and conducted around 100 searches across the states of West Bengal, Assam and Jharkhand during the investigation.

Testimonies gathered by the NIA provide details about the alleged bomb-making unit in the house, and sequence of events leading up to the explosion.

The accused have been charged under various sections of the law relating to offences of violence, conspiracy, recruitment, funding, running training camps, possession of arms and explosives, forgery and offences related to foreigners act and passport act.

Two people were killed in the blast that took place in a house at Khagragarh in Bengal's Burdwan district on 2 October 2014.

Though it was assumed to be a gas cylinder blast or explosion of crude bombs, investigations later revealed that the two people who were killed were using fairly sophisticated IEDs or Improvised Explosive Devices.