Three Indian nationals have been arrested and charged over the killing of a Sikh separatist leader in Canada, which sparked a major diplomatic row between the two countries.

Hardeep Singh Nijjar, 45, was shot dead last June by masked gunmen in a busy car park in a Vancouver suburb.

The diplomatic row escalated after Prime Minister Justin Trudeau alleged India’s government may have been involved.

Delhi strongly denied the allegation.

In announcing the arrests on Friday, Superintendent Mandeep Mooker said the three suspects were Karan Brar, 22, Kamal Preet Singh, 22, and 28-year-old Karan Preet Singh.

He said all three had been living in Edmonton, Alberta where they were arrested. They have been charged with first-degree murder, court records show, as well as conspiracy to commit murder.

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Investigators have been working with counterparts in India but the collaboration has been “rather difficult and rather challenging” for several years, they said