At least three American Sikhs were warned by the Federal Bureau of Investigation that their lives were in danger in the wake of the killing of separatist leader Hardeep Singh Nijjar in Canada, The Guardian reported on Tuesday.

Nijjar, the chief of the Khalistan Tiger Force, was shot dead in the parking lot of a gurdwara in Surrey near Vancouver on June 18. The Khalistan Tiger Force has been designated a terrorist outfit in India.

Canada has alleged that the Indian government may be behind his killing – a charge that has led to diplomatic tensions between the two nations.

New Delhi has claimed that Ottawa’s claims are “motivated” and asked it to take legal action against “anti-Indian elements” operating from its soil instead. The accusation also sparked tit-for-tat retaliation, with each country expelling diplomats. India has also indefinitely suspended visa services in Canada, citing security threats to its officials.

United States Ambassador to Canada David Cohen on Saturday said that Ottawa was informed about the possible involvement of the Indian government in the killing of the Sikh separatist leader by an intelligence-sharing alliance known as the Five Eyes. The alliance consists of the US, the United Kingdom, Australia, New Zealand and Canada.

On Tuesday, Pritpal Singh, a 69-year-old US citizen who serves as a coordinator for the American Sikh Caucus Committee, confirmed to The Guardian that he, his associate Amarjit Singh and another person were called by the Federal Bureau of Investigation just days after Nijjar’s murder.

“I was visited by two FBI special agents in late June who told me that they had received information that there was a threat against my life,” Singh had first told The Intercept on Saturday. “They did not tell us specifically where the threat was coming from, but they said that I should be careful.”

Singh added that he was later offered more specific safety instructions by the law enforcement agency. The Federal Bureau of Investigation is bound to alert citizens if it learns that their lives are in danger under legal protection known as a “duty to warn”.

“Such intimidation of Americans is a form of transnational repression by the Indian government,” Singh told The Guardian. “Transnational repression not only threatens individuals but also undermines our democratic institutions, curtails individual rights and freedoms, and challenges the national security and sovereignty of the United States.”

Amarjit Singh said that he was first alerted of a possible threat to his life on June 22 after he was returning from a protest in Washington against Prime Minister Narendra Modi during his visit to the United States.

He said the warning from the Federal Bureau of Investigation was followed by a longer in-person meeting a few weeks later. He added that he decided to go public about the warning only after Canadian Prime Minister Justin Trudeau revealed details about Nijjar’s killing.

“It was a warning,” Amarjit Singh told The Guardian. “They said no travel, just keep yourself safe.”

US State Department Spokesperson Matthew Miller on Tuesday called for a full and fair investigation into Canada’s allegations against the possible involvement of the Indian government in the case.

“They are such concerning allegations that we think there ought to be a full and fair investigation,” Miller said, according to PTI. “Canada has said it is committed to doing that, and we believe the Indian government should cooperate with it.”