A fourth person has been arrested and charged with the murder of Sikh separatist leader Hardeep Singh Nijjar last year, Canadian police said on Saturday, in a case that strained diplomatic relations with India.

Canadian police earlier this month arrested and charged three Indian men in the city of Edmonton in Alberta and said they were probing whether the men had ties to the Indian government.

The Integrated Homicide Investigation Team (IHIT) announced Saturday that Amandeep Singh, 22, has been charged with first-degree murder and conspiracy to commit murder in Nijjar’s killing.

Singh, an Indian national who resided in Brampton, Surrey and Abbotsford, was already in custody for unrelated firearms charges out of Peel, Ontario, IHIT said.

Nijjar, 45, was shot dead in June outside a Sikh temple in Surrey, a Vancouver suburb with a large Sikh population. A few months later, Canadian Prime Minister Justin Trudeau cited what he said was evidence of potential Indian government involvement, prompting a diplomatic crisis with New Delhi.

Nijjar was a Canadian citizen campaigning for the creation of Khalistan, an independent Sikh homeland carved out of India. The presence of Sikh separatist groups in Canada has long frustrated New Delhi, which had labeled Nijjar a “terrorist”.