In California and New Jersey, some Sikh temples are rushing to add security cameras and hire night patrols.

Bobbie Singh-Allen, the outspoken mayor of Elk Grove, Calif., said she had begun tempering her posts on social media that might be seen as critical of India. And Dr. Pritpal Singh, a Sikh American activist in California, said he was prepared to turn to a particularly American form of self-defense: using a gun.

“We know that there is a Second Amendment,” said Dr. Singh, who was told by the F.B.I. months ago that his life might be in danger because of his support for Sikh separatism. “We keep that in mind.”

Sikh Americans have been on edge for months since the Indian government was accused of assassinating a prominent Sikh separatist and community figure in Canada, another country where Sikhs believed their activism was protected against interference by the Indian government. Their anxieties spiked last week after federal prosecutors in New York unsealed an indictment accusing an official in the Indian government of a different assassination plot, this time one that was foiled, in the U.S.

To many Sikh Americans, the indictment — which alleged a brazen murder-for-hire plot against a Sikh separatist in New York — seemed to confirm suspicions that they had harbored for years about the Indian government keeping tabs on their actions. The vast majority of Sikhs live in India, where they are less than 2 percent of the population.

The court documents described a wad of cash, surveillance photos of the assassination target and a hired hit man who turned out to be an undercover Drug Enforcement Administration agent. U.S. government officials have confirmed that the target was Gurpatwant Singh Pannun, a New York-based Sikh separatist who holds dual American and Canadian citizenship.

“The community for a long time has understood that their dissenting voices have been silenced,” said Kiran Kaur Gill, executive director of the Sikh American Legal Defense and Education Fund, a civil rights advocacy group based in Washington. “But to see the fears of the community realized in such an extreme way with the allegations of the potential assassination against a Sikh American is deeply traumatizing.”

The first Sikh immigrants began arriving in North America in the early 1900s, working in lumber mills, railroad construction and, later, in agriculture. Most of the Sikhs in America today came after 1965 for higher education, professional jobs or to reunite with family.

They have long faced discrimination and hate attacks. Easily identifiable by the beard and turban that are a religious requirement for observant Sikh men, they are frequently mistaken for Muslims by other Americans. After the Sept. 11 attacks, there was an alarming increase in hate crimes, including the killing of Balbir Singh Sodhi, a Sikh man, outside his gas station in Arizona.

In 2012, a white supremacist shot six people dead at a gurdwara, a Sikh house of worship, in Oak Creek, Wis.. Afterward, many gurdwaras, also called gurudwaras, bolstered their security systems. And just last year, a string of hate crimes against Sikh men in New York’s Richmond Hill left the Sikh community deeply shaken.

The details in the indictment last week point to a different kind of threat, one from overseas aimed at stifling political dissent. Prosecutors said in court documents that the plot against Mr. Pannun was linked to the killing in June of Hardeep Singh Nijjar, the Sikh separatist in Canada. Both men called for the establishment of an independent nation, Khalistan, that would include parts of Punjab, a northwestern state in India where Sikhs are a majority.

The Indian Embassy in Washington did not immediately respond to an emailed request for comment. The Indian government has denied any involvement in Mr. Nijjar’s murder in Canada, which is home to the largest Sikh population outside India.

A spokesman for India’s Ministry of External Affairs said in a statement last week that the Indian government had empaneled a “high-level” committee “to look into all the relevant aspects of the matter,” an apparent reference to the plot against Mr. Pannun.

Sikh Americans have wide-ranging opinions on whether Sikhs should secede from India. The dynamics of the diaspora changed in the mid-1980s when many were fleeing anti-Sikh hostilities in India following a separatist uprising in Punjab and the 1984 assassination of the prime minister, Indira Gandhi.

At the time, Sikh militants for more than a decade had carried out a violent campaign for an independent state, including attacks that killed civilians. At the direction of Mrs. Gandhi, Indian soldiers responded by raiding the religion’s holiest place of worship, the Golden Temple in Amritsar, to remove Sikh separatists, killing hundreds, according to official figures. Sikh groups say the death toll was in the thousands.

Mrs. Gandhi was then assassinated by two of her bodyguards, who were Sikh. That prompted widespread anti-Sikh violence in northern India. Thousands of Sikhs were massacred in organized pogroms. In 1985, Khalistani separatists were accused of detonating a bomb on an Air India flight between Toronto and London, killing more than 300 people.

The Sikh secessionist movement has since faded in India. But the memories of violence and political repression of the mid-1980s are still “very much in the minds of Sikhs” in America and their American-born children, said Nirvikar Singh, a professor of economics at the University of California, Santa Cruz, and co-author of “The Other One Percent: Indians in America.”

“I think what a lot of Sikhs feel is that there was no justice,” Mr. Singh said.

Sikh Americans have grown more wary of the Indian government under Prime Minister Narendra Modi, a Hindu nationalist who, they say, has portrayed the Sikh separatist movement at home and abroad as more serious a threat than it is.

For Mr. Modi, amplifying the threat may help to bolster his status as a political strongman ahead of a national election next year. The Sikhs of Punjab have been a thorn in his side since they led protests that defeated his plans for reforming Indian agriculture in 2021.

Like most Sikhs in India, many Sikh Americans believe that Punjab should remain under Indian government control but have greater autonomy. Some support the status quo. Others — a small minority, community leaders say — are adamant that an independent Sikh homeland is necessary for Sikhs to flourish.

India has declared Sikh separatists abroad to be terrorists, and Indian officials have accused their counterparts in Western nations of being too lax when Khalistan supporters have vandalized Indian embassies and consulates and threatened Indian diplomats. Officials in Western countries see the separatists as activists who at times have crossed a line with calls to violence but whose right to free speech is protected by law.

Some Sikh American leaders want a stronger public rebuke of India from the Biden administration, which has responded behind the scenes as it tries to shore up an alliance with the Modi-led Indian government as a wedge against China and Russia.

“As a U.S. citizen, I demand to see an investigation,” said Ms. Singh-Allen, the mayor of Elk Grove, Calif. “This is unacceptable that a foreign government can threaten U.S. citizens. Would this be elevated if it was Russia? We need to treat it with that same level of concern.”

Deep Singh, executive director of the Jakara Movement, a nonprofit that works in Punjabi Sikh communities in California, said that the indictment had already sent a chill through the community. “We had folks that were signing onto a letter seeking some kind of congressional redress and later calling and saying, ‘Please remove our names, we’re too worried about our family in India,’” he said.

Still, some Sikh activists have vowed to press on with their advocacy for an independent state, including Sikhs for Justice, the group for whom Mr. Pannun, the assassination target, serves as general counsel.

Others in the community have vowed to keep lobbying local institutions in Western countries to pass resolutions that officially recognize the violence against Sikhs in 1984 as a genocide.

“Sikhs are warriors,” said Dr. Pritpal Singh, the Sikh activist. “It’s in our genes, it’s in our blood.”