Sikh activist Hardeep Singh Nijjar was a long-time campaigner for a separate homeland for people of his faith, whose murder in June near Vancouver has now sparked a diplomatic storm between India and Canada.

Nijjar was wanted by Indian authorities for alleged terrorism offences and conspiracy to commit murder, which he had denied to Canadian media.

Little is known about his early life. He was born in 1977 in India's Punjab state, the spiritual heartland of the Sikh religion where nearly 60 percent of the population follow the faith.

While he was still a child, Punjab was home to a violent insurgency prosecuted by extremists seeking to carve out a sovereign Sikh nation from the state.

Nijjar arrived in Canada at the age of around 20 in 1997, after the insurgency had been brought to heel by Indian security forces.

Canadian outlet Global News reported, citing Nijjar's immigration records, that he used a fraudulent passport to enter the country, which is home to the largest community of Sikhs outside of India.

He later ran a plumbing business and was president of a gurdwara, or Sikh temple, near Vancouver.

At the same time, he also became involved with groups campaigning for "Khalistan", the proposed name for a Sikh homeland.

Nijjar was reportedly involved in staging referendums for emigre Sikhs to demonstrate their support for the establishment of Khalistan.

Despite living in Canada, Nijjar was accused by Indian authorities of sponsoring numerous attacks, killings and violent acts back in his home country.

India's National Investigation Agency designated him a "terrorist" and last year offered a reward of one million rupees ($12,000) for information leading to his arrest.

News outlet India Today said Nijjar had been accused of involvement in a 2007 bombing that killed six people and injured 42 others in the city of Ludhiana, not far from his hometown.

India has often complained to foreign governments, including Canada, about the activities of Sikh hardliners among the Indian diaspora who it says are trying to revive the 1980s Punjab insurgency.

Prime Minister Justin Trudeau visited Punjab in 2018, where the state's chief minister demanded action against Nijjar and eight other Sikh separatist leaders.

- Murder and diplomatic storm -

Nijjar was shot dead by two unknown masked gunmen in June outside the temple he presided over near his home in Surrey, British Columbia.

After his death, the World Sikh Organization of Canada said he had been warned by the country's spy agency about threats against him.

Members of Canada's Sikh community quickly pointed the finger at India, with several hundred demonstrating outside the Indian consulate in Toronto the following month.

Canada's government gave public endorsement to those claims on Monday, when Trudeau told an emergency parliamentary session that his government had "credible allegations" linking Indian agents to Nijjar's slaying.

Foreign Minister Melanie Joly also announced the expulsion of a top member of the Research and Analysis Wing (RAW), India's foreign intelligence agency, who had been serving as a diplomat in Canada.

India rejected allegations of its involvement as "absurd" and responded by ordering a Canadian diplomat to leave New Delhi.

The tit-for-tat expulsion reflected India's "growing concern" at the involvement of Canadian diplomats in "anti-India activities", the country's foreign ministry said.