It was a mild Sunday evening in Surrey, a city near Vancouver, British Columbia, and Hardeep Singh Nijjar was ready to drive home after spending the day at his Sikh temple. He told a friend he thought he was being followed, but that night he just wanted to celebrate Father’s Day with his family.

Mr. Nijjar was driving his truck out of the parking lot when he was ambushed. Two masked gunmen unleashed a burst of gunfire and then fled in a getaway car. Mr. Nijjar was dead.

The murder on that day in June became part of a series of events that would ricochet around the world, with federal agents in the United States working furiously behind the scenes to unravel an international murder plot they believed was orchestrated by someone within the Indian government. The geopolitical implications were enormous, and the clock was ticking: the next assassination planned was of someone on American soil.

That explosive tip had reached the Drug Enforcement Administration through an unexpected route, according to court records and interviews with people familiar with the investigation — accounts that, taken together, provide a detailed picture of how the episode unfolded.

What followed was an elaborate sting operation involving an undercover DEA agent posing as a hit man and a stack of $15,000 in cash bills, overseen by an extensive team of investigators who had access to private text messages between Indian nationals living in India .

This week, federal prosecutors in Manhattan unsealed charges against Nikhil Gupta, a native of India accused of masterminding the murder plot in the United States. Officials say the plot targeted a prominent American Sikh activist living in New York City who was a longtime colleague of Mr. Nijjar, also an outspoken Sikh leader.

Mr. Gupta was arrested in the Czech Republic on June 30 and is awaiting extradition to the United States, a spokesman for the Czech Justice Ministry said. Mr. Gupta’s lawyer did not respond to requests for comment.

In a vaguely worded response to the allegations, a spokesperson for India’s Foreign Ministry said the Indian government had opened an investigation on November 18 after hearing from the United States about “organized criminals, arms smugglers, terrorists and others.” The Indian government has denied any involvement in Mr Nijjar’s killing in Canada.

Before the murder plot came into the public eye, Mr. Gupta was already a household name among some law enforcement officials in the United States, suspected of taking part in the sale of heroin and cocaine, according to a person familiar with the investigation. In conversations recorded by prosecutors, he had discussed his involvement in the international drug and arms trade.

But when Mr. Gupta called one of his colleagues in the drug trade in late May, he was looking for something completely different.

Mr. Gupta asked the man if he happened to know anyone who could be hired to carry out a murder in the United States.

What Mr. Gupta didn’t realize was that the drug trafficker was actually an informant working for the DEA

The DEA had used the informant in another Colombian drug trafficking case. But Mr. Gupta’s efforts took the investigation in a new direction, people familiar with the investigation said.

Mr. Gupta became involved in the plot a few weeks earlier, according to prosecutors, after an Indian government official recruited him to arrange an assassination attempt in New York. Prosecutors said the official, who was not named in the indictment, described working in a government job responsible for intelligence and security management.

The suspected target was Gurpatwant Singh Pannun, an outspoken supporter of a Sikh independence movement that India has long seen as a threat. Like Mr. Nijjar, he pushed for a sovereign state carved out of India for his religious minority group, a dispute rooted in decades of history. It was not clear whether the official in the case acted alone or with the blessing of others in the Indian government.

To win Mr. Gupta’s help, the official promised to resolve a criminal case hanging over him in India, the indictment said. It was not clear what the charges were.

Within a few weeks, the official seemed to make good on the promise. The official assured Mr. Gupta that the matter “has already been resolved” and that “no one will ever bother you again,” according to the complaint. Prosecutors said the official even offered to introduce Mr. Gupta to a deputy police commissioner in India.

With his case dismissed in India, Mr. Gupta moved forward with the scheme, prosecutors said.

After the DEA learned that Mr. Gupta was looking for a hit man, investigators came up with an idea: A DEA agent would go undercover and play the role of a hitman.

The DEA informant introduced Mr. Gupta to the fake assassin and they agreed on a price for the killing: $100,000.

Mr. Pannun, the man in the crosshairs, had been a lawyer for Mr. Nijjar, the murder victim, in Canada. Both men were outspoken critics of India’s leader, Prime Minister Narendra Modi.

In the weeks that followed, the indictment said, Mr. Gupta passed information from the Indian government official to the assassin, including Mr. Pannun’s home address in New York and details of his daily routine. Prosecutors say Mr. Gupta suggested during a video call that the DEA informant could more easily lure Mr. Pannun by pretending to seek legal advice from him.

In early June, the informant asked Mr. Gupta for details about the cash and requested an advance.

The Indian official initially told Mr. Gupta that an upfront fee was impossible and that the full $100,000 payment would be made within 24 hours after the job was completed, the complaint said. But the official eventually relented and named an employee who would help arrange the funds.

On June 9, the hitman met another associate of Mr. Gupta in his car, who showed up with $15,000 in cash. The employee handed over a huge stack of folded $100 bills.

But throughout the planning, the indictment said, there was one key stipulation by the Indian official: the killing could not take place during high-level visits between U.S. and Indian officials in late June, to fit the timeframe in which President Biden would to welcome. Modi to the White House for a state visit.

Given Mr. Pannun’s public profile, Mr. Gupta feared there could be protests and geopolitical fallout if Mr. Pannun were killed in the United States while Mr. Modi was visiting.

In mid-June, Mr. Gupta told the DEA informant in a phone call that there was also a “major target” in Canada.

It is not clear whether U.S. officials alerted Canadian officials. But six days later, on June 18, Mr. Nijjar was murdered outside his temple.

Hours after the killing, the indictment said, the Indian government official sent a video of Mr. Nijjar’s bloodied body slumped in his car to Mr. Gupta, who passed the video on to his associates in New York. Mr. Gupta confirmed to the DEA informant that Mr. Nijjar was the Canadian target he had previously mentioned.

Mr. Gupta told the Indian official that he wished he had personally carried out Mr. Nijjar’s killing and asked permission to “go to the field,” according to the indictment. The official told him to lay low and said it was best to stay behind.

Their plan to be cautious around Mr. Modi’s visit flew out the window. Two days after Mr. Nijjar’s killing, the Indian official, according to the indictment, sent Mr. Gupta a news article about Mr. Pannun, saying it was now a “priority.”

Mr. Gupta relayed the message to the man he thought would pull the trigger. Mr Gupta said there is now “no need to wait” to kill Mr Pannun, adding: “We have so many targets.” He said Mr Nijjar was No. 3 or No. 4 on the list.

If Mr. Pannun was in a meeting with others, “put everyone down,” Mr. Gupta said, according to the complaint.

The pressure mounted and Mr. Gupta panicked. He demanded regular updates from his New York employees. Mr. Gupta said they had to complete four jobs by June 29, including three people in Canada.

The Indian official kept a close eye on the progress of the operation. In late June, the official messaged Mr. Gupta to say that Mr. Pannun was not home — intelligence, the official said, that came from an unnamed “boss,” according to the indictment.

On June 29, Mr. Gupta learned that Mr. Pannun was finally at his home and told the hitman to commit the murder, prosecutors said. Mr. Gupta urged him to try “to get this done when you have the footage and when you are sure.”

But there had never been an assassin before. The plot that prosecutors say had haunted Gupta for more than a month could go no further.

The next day, Mr. Gupta flew from India to the Czech Republic for unclear reasons.

As soon as he arrived, he was arrested by the Czech police, who were waiting for him at the request of the American authorities.

A spokesperson for the Czech Justice Ministry said a decision on Mr. Gupta’s extradition would be made in the coming weeks.

After the case became public this week, Mr. Pannun said in an interview that he was not surprised by the plot against him and vowed to remain committed to his activist efforts.

“I am not afraid of physical death,” Mr. Pannun said. “We live in the home of the brave and the land of the free in America.”

Julian E. Barnes, Kim Barker, Jesse McKinley, Barbora Petrova and Ian Austen contributed reporting.