The fourth was Robert Flanagan, 24, who did not know the others before roughly two weeks ago, his lawyer said, when Mr. O’Keefe gave a speech for the Pelican Institute for Public Policy, a libertarian organization in New Orleans for which Mr. Flanagan works a few hours a week. Until then, Mr. Flanagan, a star athlete and son of a federal prosecutor, had not been known by friends to be particularly provocative in his conservatism, though he had been sharply critical of Ms. Landrieu on the institute’s blog.

Image The house in New Orleans where three of the arrested men stayed. Credit... Patrick Semansky/Associated Press

And then there was Ben Wetmore, 28, who was not arrested but who allowed Mr. Dai, Mr. O’Keefe and Mr. Basel to stay at his house in New Orleans this month. The authorities have not indicated that Mr. Wetmore, a Loyola law student, was connected to the incident at Ms. Landrieu’s office, but he has nonetheless played a vital role in Mr. O’Keefe’s career, as well as that of Mr. Basel and other activists.

Mr. Wetmore helped introduce many of the activists to one another and inspired them through his take on attention-grabbing tactics. His often behind-the-scenes role was detailed in a trail he left on the Internet, as well as in several interviews.

“Benjamin Wetmore: a mentor of mine; a genius,” Mr. O’Keefe said during an interview with The New York Times in September, after the Acorn videos were released. “He said, ‘Take on the politically correct crowd on campus, satirically.’ ”

Mr. O’Keefe declined several interview requests, and Mr. Wetmore responded to an e-mail message by sending photographs of Jayson Blair, a reporter for The New York Times who resigned after admitting to plagiarism and fabrication. Mr. Dai, Mr. Basel and Mr. Flanagan could not be reached for comment. (The four men arrested were freed on bail, awaiting a pretrial hearing.)

The partnership between Mr. O’Keefe and Mr. Wetmore appears to have started in earnest in 2004.

As a philosophy major at Rutgers University, Mr. O’Keefe came to believe that conservative-leaning students were being force-fed a diet of academic liberalism. As he put it at the time, they were “drowned in relativism, concepts of distributive justice and redistribution of wealth.”

He and some friends started an alternative conservative publication called The Centurion with $500 from the conservative Leadership Institute’s Balance in Media grant program, which was overseen at the time by Mr. Wetmore. The institute, founded in 1979, is based in Arlington, Va., and is best known for training campus conservatives to influence public policy.