Ravinder Gosain was shot dead in Ludhiana on October 17.
The NIA on Thursday took over probe into the killing of an RSS worker in Punjab, the agency said.
The National Investigation Agency or NIA has been entrusted with probing the killing of Rashtriya Swayamsevak Sangh (RSS) worker Ravinder Gosain by the Union home ministry, after which the agency registered an FIR under various sections of the IPC and the Unlawful Activities (Prevention) Act, an agency spokesperson said.
Mr Gosain, the 'Mukhya Shikshak' of the Mohan shakha of the RSS, was shot dead outside his residence in Ludhiana's Gagandeep Colony in the morning of October 17 by two unidentified assailants riding a motorbike.
The Punjab police, which was earlier probing the case, arrested two people -- Ramandeep Singh and Hardeep Singh -- a few days after the incident.
"During their examination, these accused persons have admitted their involvement in the previous eight incidents of murder or attempted murder in Punjab which have happened since January, 2016.
"Targets in these incidents generally were members of the RSS and Hindu organisations," the NIA spokesperson said.
Besides this, he said, in July, 2017, they also murdered a Christian pastor, Sultan Masih, in Ludhiana.
"It has also come to light that the above mentioned conspiracy to destabilise Punjab has been hatched by Sikh extremist elements and others located in various parts of the world including the United Kingdom, France, Italy, United Arab Emirates and Pakistan," he said.
He added that investigation conducted so far has revealed channelling of funds from foreign countries for execution of these incidents.
The conspiracy also included an element of ideological brainwashing and incitement of the accused on religious grounds by their mentors settled abroad.