USC athletic director Mike Bohn resigned from his position, he and the school announced Friday.

Bohn's resignation comes at a surprising time, less than four years after he was hired from Cincinnati to replace Lynn Swann, who resigned from the job after three years.

Bohn mentioned "ongoing health challenges" in a statement issued to the Los Angeles Times and told ESPN's Pete Thamel that it was an amicable split with the university.

"It's a great time with USC's emergence into the Big Ten for new leadership to move the program forward," Bohn said.

But the Times reported Friday that Bohn's resignation came shortly after the newspaper asked him and the school about internal criticism of his management of the athletic department. According to the Times, four current and former USC employees said staffers inside the department had raised concerns about Bohn.

As USC's athletic department and football team needed revamping, the 62-year-old Bohn arrived as the first athletic director in the previous 25 years without any prior connection to the school.

The biggest personnel move during his tenure was the firing of football coach Clay Helton and subsequent hiring of Lincoln Riley away from Oklahoma last year. The move produced immediate results: USC went from 4-8 to 11-3 and was one win away from the College Football Playoff while boasting the Heisman Trophy winner Caleb Williams. This year's team is expected to compete for a CFP spot again with Williams under center.

But perhaps the more consequential decision ushered under Bohn's management was the orchestrating of USC's move from the Pac-12 to the Big Ten alongside UCLA. The sport-altering move, which was announced last year and will go into effect next year, not only put USC in a conference alongside Michigan and Ohio State but also allowed it to be part of a seven-year, $7 billion media rights deal between the Big Ten and broadcasters Fox, CBS and NBC.

"Over the last four years, the USC athletics department has transformed into a national powerhouse," university president Carol Folt wrote in a letter to the USC community. "I am committed to ensuring we have the right leadership in place to achieve our goals. As part of that commitment and as we prepare to move to the Big Ten, we conducted a thorough review of the athletics department, including its operations, culture, and strategy. Having built a strong foundation over the last few years, now is the time for new direction grounded in our values and in expertise needed to fulfill our aspirational vision for Trojan athletics."

The Times reported that USC retained Gina Maisto Smith, a Philadelphia-based attorney from Cozen O'Connor, to conduct the internal review that Folt mentioned in her letter.

The football team wasn't the only program heading in the right direction under Bohn. The men's basketball team made three straight NCAA tournament appearances and just signed LeBron James' son, Bronny. Bohn also hired Lindsay Gottlieb away from the Cleveland Cavaliers to coach the women's basketball team, which advanced to the NCAA tournament this spring in her second season.

Along with USC and Cincinnati, Bohn previously served as the athletic director at Idaho, San Diego State and Colorado.