“During the air battle, the F-16 aircraft demonstrated their high efficiency,” Ukraine’s General Staff said in a statement. The Ukrainian Air Force said Moonfish helped down three cruise missiles and one drone on Aug. 26—his final mission.

“During the approach to the next target, contact with one of the aircraft was lost,” the General Staff said.

On Aug. 26, Ukrainian president Volodymyr Zelenskyy touted the effectiveness of the F-16s in their first contact with enemy targets but made no mention of any losses to aircraft or pilots.

It is unclear what caused the crash. The Ukrainian military said it is investigating. A member of the Ukrainian parliament, Marina Bezuhla, said the incident may have been a case of friendly fire. Moonfish’s F-16 was likely shot down by another American-made weapon, a Patriot surface-to-air missile system, “due to a lack of coordination between units,” Bezuhla wrote on social media. No evidence has been presented for that theory so far.

A memorial for Mes was held on Aug. 29.

“We lost a reliable friend and a strong warrior,” the Ukrainian Air Force said in a statement. A pair of MiG-29s flew over the ceremony. Mes was posthumously promoted to colonel.

“F-16 fighters of the Armed Forces of Ukraine were used together with anti-aircraft missile units during the repulse of a missile attack by the Russian Federation on the territory of Ukraine,” the General Staff statement said. It said a committee was investigating the crash.

Deputy Pentagon Press Secretary Sabrina Singh declined to provide details of the crash and said the U.S. was not involved in the crash probe.

“The United States has not been asked to participate in any type of investigation to look into this incident,” Singh said. “Broadly speaking, combat aviation is incredibly complex, and we’ve been very proud to train some of the pilots here in the United States. … Every day that they fly those aircraft, these are brave men and women going up there to defend their skies and to defend their country.”

Moonfish is the second high-profile Ukrainian pilot who advocated for Ukraine to get F-16s to be killed. Andriy “Juice” Pilshchykov and two other pilots were killed in an accident involving trainer aircraft in 2023. Moonfish and Juice appeared together in the media, including in the United States, and lobbied Congress to pressure the Biden administration to authorize the transfer of F-16s.

Moonfish “was the very first trained F-16 pilot for Ukraine and carried that pride even into his death,” wrote former Rep. Adam Kinzinger (R-Ill.), who met with the two pilots in 2022.

“His skills as a pilot were unmatched, his instincts razor-sharp,” Kinzinger added. “He had a natural talent for flying, but it was his heart that set him apart. Moonfish flew not just for victory, but for justice—for the children of Ukraine, for the future of his nation.”

The Ukrainian Air Force, also known as the PS ZSU, also celebrated the pilot.

“Oleksiy saved Ukrainians from deadly Russian missiles, unfortunately at the cost of his own life,” the PS ZSU said. “Oleksiy Mes was a strong and loyal soldier, a high-class pilot, a leader on earth and in the sky, a good friend, a loving son, father, husband, and patriot of his country.”

Airpower experts and U.S. officials have cautioned that Ukraine’s second-hand F-16s, publicly unveiled Aug. 4, will not provide Kyiv with air superiority in the near term. The fighters have been spotted carrying AIM-120 AMRAAM and AIM-9 Sidewinder air-to-air missiles, and the U.S. has already provided the Ukrainian Air Force with JDAM Extended Range guided bombs and HARM anti-radiation missiles for its legacy fleet of MiGs and Sukhoi jets.

The F-16 just celebrated its 50th birthday. The venerable, multirole fourth-generation fighter has been heavily upgraded over the years and is still in production. Denmark and the Netherlands are donating Ukraine’s first jets, and Norway and Belgium have also promised to provide Ukraine with F-16s—some 80 total aircraft. The U.S. is not providing Ukraine with any Vipers, but the U.S. Air Force’s 350th Spectrum Warfare Wing recently announced that it helped enhance the electronic warfare capabilities of the foreign jets Ukraine is receiving and plans to continue providing support.

Zelenskyy has said Ukraine needs more F-16s and permission to engage targets in Russia to protect Ukraine’s skies.