On a warm July night two years ago, Moninder Singh received a chilling message from special federal agents who showed up at his house in British Columbia: You are being formally warned that there is an imminent threat to your life. Avoid public spaces. Enhance security at home.

The first person he called — a friend and fellow activist in a campaign promoting an independent Sikh homeland carved out of India — had just gotten the same ominous warning.

A year later, that friend, Hardeep Singh Nijjar, was dead.

Murder at the Temple

Mr. Nijjar was gunned down in June 2023 by masked men outside a Sikh temple he led in British Columbia. The Canadian government blamed the Indian government for the killing, setting off an extraordinary diplomatic rift.

Now, Prime Minister Justin Trudeau and Canadian law enforcement officials have painted an even darker picture: Mr. Nijjar’s assassination, they said, was part of a broader criminal campaign run by India that targeted Sikhs on Canadian soil and included harassment, intimidation, extortion and the killing of at least one other person.

The campaign was orchestrated, officials said, by India’s ambassador in Canada and other diplomats out of the embassy in Ottawa and consulates in Toronto and Vancouver.