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 had told a friend that he thought he was being followed, but that night, he was just eager to celebrate Father’s Day with his family.

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

The murder that day in June became part of a chain of events that would ricochet around the world, with federal agents in the United States working furiously behind the scenes to untangle an international assassination plot that they believed was directed by someone inside India’s government. The geopolitical implications were huge, and the clock was ticking: The next murder being planned was for someone on U.S. soil.

That explosive tip had come into the Drug Enforcement Administration through an unexpected avenue, 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 D.E.A. agent posing as a hit man and a wad of $15,000 in cash bills, overseen by a sprawling team of investigators who were able to access private text messages between Indian nationals living in India.

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