Former UFC heavyweight champion Brock Lesnar has been notified of a potential violation of the promotion’s anti-doping policy from an out-of-competition drug test administered June 28.

Lesnar is among the most popular athletes in both mixed martial arts and professional wrestling. He beat Mark Hunt by decision at UFC 200 in Las Vegas last weekend, handily winning his first MMA fight in nearly five years.

“We will get to the bottom of this,” Lesnar said Friday in a statement provided exclusively to The Associated Press.

The AP also obtained copies of three letters from the U.S. Anti-Doping Agency to Lesnar, informing the fighter that he had passed tests on three samples submitted over a nine-day period in mid-June, mere days before the sample with the potential violation was collected.