In a tweet sent out Wednesday afternoon, Brooks Koepka announced his withdrawal from next week’s U.S. Open.

The 30-year-old didn’t give any reason for pulling out, but he has struggled with a left knee issue since last year. He had a procedure on it last September and then reinjured his knee in October.

“Unfortunately, I have decided to withdraw from next week’s U.S. Open,” read Koepka’s tweet. “I'm looking forward to getting healthy and competing at 100% again very soon.”

Koepka won the American championship in 2017 and ’18, then finished runner-up last year to Gary Woodland.

The withdrawal continues the mystery of his injury or, possibly, injuries. In the restart to the PGA Tour season, Koepka hinted that there were issues but did not disclose any specifics. At the PGA Championship, he was getting stretched out by a trainer on the course during the middle of the tournament for what later he said was a hip problem.

After missing the cut at the Wyndham Championship, the four-time major winner once again admitted to a physical ailment without any indication of exactly what it was. His only notice was that it was limiting his ability to swing the club properly

“This is physical, yeah. I know how to do it. I can do it, I just physically can’t do it,” Koepka said at the time. “My golf swing’s fine. If I can physically do it, then yes, everything’s fine.”

His play since the restart had some bright moments, like a tie for second at the World Golf Championship-FedEx St. Jude Invitational, but he withdrew from the Northern Trust, the first event of the FedEx Cup playoffs, ending his season.