Sergeant Enos declined to comment, saying his O.S.I. detachment commander, Lt. Col. Vasaga Tilo, had ordered him not to speak to the news media or the academy’s leadership about the program. But in a 12-page letter sent to members of Congress in May, he said his first case at the academy had involved a female cadet who had been sexually assaulted. She told Sergeant Enos that her attacker had also assaulted other women. But, he recalled her saying, there was nothing the sergeant could do because the cadet was on the football team, which meant he could “get away with murder.”

“I felt sick to my stomach that someone could think this way,” Sergeant Enos wrote in the letter, a copy of which was obtained by The New York Times. “I told the female cadet I would do everything I could to change this culture.”

He decided to try using an informant and, within weeks, recruited Mr. Thomas.

Sergeant Enos directed Mr. Thomas to go after the football team, believed to be a hotbed of misconduct. Mr. Thomas began hanging out with the beer drinkers at parties, taking photos on his phone and sometimes wearing recording devices. He met Sergeant Enos almost daily behind the academy’s B-52 bomber to file secret reports.

“Eric Thomas was how I found out what was truly going on inside the football team,” the sergeant wrote.

In January 2012, based primarily on Mr. Thomas’s findings, the Office of Special Investigations began its largest law enforcement action ever at the academy, interrogating 50 cadets in one day, most of them football players. It called the crackdown Operation Gridiron, records show. Officials at O.S.I. headquarters in Virginia, the academy superintendent and the secretary of the Air Force were briefed regularly, Sergeant Enos wrote in his letter.

Based in part on the work of Mr. Thomas and other informants, three football players were convicted in 2013 of sexual assault, including a star linebacker, Jamil Cooks, Sergeant Enos said. Other players were expelled for drug use.

In the wake of those convictions, reports of sexual assault at the academy — a statistic that Air Force leaders regularly cite as a barometer of confidence in the system — almost doubled. This year, after the informant program ended with no further convictions, reports fell by half, according to academy officials.