The National Investigation Agency on Friday said it has arrested the alleged key conspirator behind the Ludhiana court bomb blast that took place in December last year, PTI reported.

The accused man, Harpreet Singh, was arrested at the Indira Gandhi International Airport in New Delhi. The agency had declared a reward of Rs 10 lakh for Singh.

One person was killed and five others were injured in the blast. The police had said that they suspect that the man who died on the second-floor toilet of the court complex was trying to assemble or plant an explosive device.

A National Investigation Agency spokesperson said that Singh, along with an associate of Lakhbir Singh Rode, the Pakistan-based chief of the International Sikh Youth Federation, were the main conspirators in the blast.

The International Sikh Youth Federation is a banned organisation that seeks to establish an independent homeland for the Sikh community.

“Acting on the directions of Rode, Singh coordinated the delivery of the custom-made Improvised Explosive Device, which had been sent from Pakistan to his India-based associates, that was used in the blast,” the spokesperson said, according to PTI. “Further investigations in the case are in progress.”

Singh was also involved and wanted in various cases, including smuggling of explosives, arms and narcotics, the spokesperson added.

The investigating agency has already arrested three more persons – Surmukh Singh Sammu, Dilbagh Singh Baggo and Rajanpreet Singh – in connection with the blast, reported The Indian Express.

Of these, Rajanpreet Singh had allegedly given the IED to Gagandeep Singh Gaggi, a dismissed Punjab Police official who had died trying to allegedly plant or assemble the explosive.