On an evening in February when the Brooklyn human-rights activist Masih Alinejad was scheduled to speak at Fairfield University, in Connecticut, two men from New York visited the campus. They had been sent there as part of an Iran-backed plot to murder her, federal prosecutors said last week.

It was one of several attempts by Iran’s government to kidnap or kill Ms. Alinejad, who has criticized the country’s repression of women. In a criminal complaint made public on Friday, federal prosecutors in the office of the U.S. attorney for the Southern District of New York described a monthslong surveillance operation by the two men, who were charged in the murder-for-hire-plot along with a third man who was giving them instructions.

That man was working as an Iranian operative, prosecutors said, and he told federal authorities that he had been assigned to carry out a plot to assassinate Donald J. Trump before the presidential election.

“Actors directed by the government of Iran continue to target our citizens, including President-elect Trump, on U.S. soil and abroad. This has to stop,” the U.S. attorney for the Southern District, Damian Williams, said in a statement.

The day after the Fairfield University event, one of the New York men, Carlisle Rivera, received a money transfer of about $560, according to the complaint. In the following weeks and months, Mr. Rivera, 49, and his associate, Jonathan Loadholt, 36, staked out Ms. Alinejad’s home in Brooklyn, taking pictures and sending them to the third man, Farhad Shakeri, according to federal authorities.

In addition to the plot to kill Ms. Alinejad, Mr. Shakeri, 51, told federal authorities that he was assigned in September by the Islamic Revolutionary Guards Corps of Iran to carry out the plan to assassinate Mr. Trump. Mr. Shakeri said that in October, he was directed to come up with a plan within seven days to kill Mr. Trump.

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