The US has foiled a plot to kill Sikh separatist leader Gurpatwant Singh Pannun on US soil, a US official familiar with the matter said on Wednesday.

U.S. authorities raised concerns with New Delhi that the Indian government may have been aware of the plot, said the official, who was not authorized to comment on the sensitive matter.

The official declined to comment on when and how U.S. officials became aware of the plot

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how the alleged assassination attempt went off the rails. The FBI is investigating the matter, the official said.

Spokespeople for the FBI and Justice Department declined to comment Wednesday.

The revelation follows Canadian Prime Minister Justin Trudeau’s revelation in September of what he said were credible allegations that the Indian government may have had links to that country’s killing of Sikh activist Hardeep Singh Nijjar. India rejected the accusation as absurd, but Canada expelled a top Indian diplomat during the investigation.

The alleged assassination plot was first reported by the Financial Times, which said the US informed some allies of the alleged plot.

The official who spoke to the AP added that U.S. authorities at the highest levels of the Indian government have raised concerns about the plot, and officials in New Delhi indicated they were treating the matter seriously.

We treat this matter with the utmost seriousness, and the issue has been raised by the US government with the Indian government, including at the highest levels. Indian counterparts expressed surprise and concern, National Security Council spokesperson Adrienne Watson said in a statement.

They stated that this type of activity was not their policy. Based on conversations with senior U.S. government officials, we understand that the Indian government is further investigating this matter and will have more to say in the coming days. We have expressed our expectation that anyone found responsible should be held accountable,” the statement said.

Law enforcement officials have repeatedly warned about what they say is an alarming trend of foreign governments seeking to harass, stalk or intimidate dissidents and political opponents in the US.

India’s Ministry of Foreign Affairs said in a statement that the US has raised information about a link between organized criminals, arms smugglers, terrorists and others during recent talks. The statement described the information as a source of concern for both countries and said India is taking it seriously.