United States authorities thwarted a plot to kill a Sikh separatist in the United States and issued a warning to India over concerns the government in New Delhi was involved, a senior Biden administration official said.

The U.S. is treating the plot with utmost seriousness and has raised the issue with the Indian government “at the senior-most levels,” the White House said on Wednesday.

The Financial Times first reported the plot.

White House spokesperson Adrienne Watson said Indian officials expressed “surprise and concern” when they were informed about the incident.

“We are treating this issue with utmost seriousness, and it has been raised by the U.S. government with the Indian government, including at the senior-most levels,” Watson said.

“They stated that activity of this nature was not their policy … We understand the Indian government is further investigating this issue and will have more to say about it in the coming days. We have conveyed our expectation that anyone deemed responsible should be held accountable,” she said.

Gurpatwant Singh Pannun, who says he is a dual citizen of the United States and Canada, was the target of the foiled plot, according to the senior administration official.

News of the incident comes two months after Canada said there were “credible” allegations linking Indian agents to the June murder of a Sikh separatist leader, Hardeep Singh Nijjar, in a Vancouver suburb, something India has rejected.

India’s anti-terror agency filed a case against Pannun on Monday stating that he warned flag carrier Air India passengers in video messages shared on social media this month that their lives were in danger.

The issue is a highly delicate one for the Biden administration, which has been working to develop close relations with India given shared concerns about China’s rising power.

Indian foreign ministry spokesperson Arindam Bagchi said when asked about the FT report that Washington had shared “some inputs” that were being “examined by “relevant departments.”

Bagchi said the inputs pertained to the “nexus between organized criminals, gun runners, terrorists and others.”

“India takes such inputs seriously since it impinges on our own national security interests as well,” he said.

The FT said its sources did not say if the U.S. protest to India resulted in the plot being abandoned, or if it was foiled by the FBI. It said the protest was registered after Indian Prime Minister Narendra Modi was welcomed on a state visit by President Joe Biden in June.

Biden is currently vacationing on the Massachusetts island of Nantucket for the U.S. Thanksgiving holiday.

Apart from the diplomatic warning to India, U.S. federal prosecutors have also filed a sealed indictment against at least one suspect in a New York district court, the FT said.

The U.S. Justice Department declined to comment.

Pannun, like Nijjar, is a proponent of a decades-long but now fringe demand to carve out an independent Sikh homeland from India called Khalistan, a plan New Delhi sees as a security threat due to a violent insurgency in the 1970s and 1980s.

India’s National Investigation Agency (NIA) registered a case against Pannun under charges related to terrorism and conspiracy, among others. It stated he threatened in video messages to not let Air India operate anywhere in the world.

The case comes against the historical backdrop of a bombing in 1985 of an Air India aircraft flying from Canada to India that killed 329, and for which Sikh militants were blamed.

Pannun told Reuters on Tuesday that his message was to “boycott Air India not bomb.”

He told Reuters on Wednesday he would let the U.S. government respond “to the issue of threats to my life at the American soil from the Indian operatives.”

“Just like Canadian citizen Hardeep Singh Nijjar’s assassination by the Indian agents on Canadian soil was a challenge to Canada’s sovereignty, the threat to (an) American citizen on American soil is a Challenge to America’s sovereign(ty),” he said.

Pannun is the general counsel of Sikhs for Justice, which India labelled an “unlawful association” in 2019, citing its involvement in extremist activities. Pannun was listed as an “individual terrorist” by India in 2020.