In a letter to Turkish Defense Minister Hulusi Akar, acting Defense Secretary Patrick Shanahan gave Turkey until July 31 to discontinue its purchase of the S-400 system or the United States will stop training Turkish pilots on the F-35, proceed with the F-35 development program without Turkish input, and find new manufacturers for jet parts solely made in Turkey. Turkish officials told Hurriyet Daily News that there would be no change in the acquisition of the S-400 in response to the letter. The U.S. "wind down" could take place in early 2020 should Turkey proceed with the S-400 purchase.