KYIV, Ukraine (AP) — U.S. experts have joined a Ukrainian investigation into why an F-16 warplane that Ukraine received from its Western partners crashed earlier this week during a Russian bombardment, the country’s air force commander said Friday.

Commander Mykola Oleshchuk said on Telegram that “a detailed analysis is already being conducted” into why the fighter jet went down on Monday, when Russia launched a major missile and drone barrage at Ukraine.

“We must carefully understand what happened, what the circumstances are, and whose responsibility it is,” Oleshchuk wrote in the post.

The crash was the first reported loss of an F-16 in Ukraine, where they arrived at the end of last month. At least six of the warplanes are believed to have been delivered by European countries.

Military analysts say the planes won’t be a game-changer in the war, given Russia’s massive air force and sophisticated air defense systems. But Ukrainian officials welcomed the supersonic jets, which can carry modern weapons used by NATO countries, for bringing an opportunity to hit back at Russia’s air superiority.

Russia’s army is making slow but gradual progress in its drive into eastern Ukraine, while Ukraine’s forces are holding ground in the Kursk border region of western Russia after a recent incursion.

Oleshchuk, the Ukrainian air force chief, directed scathing criticism at a lawmaker who is deputy head of the Ukraine parliament’s defense committee for her claims that the F-16 was downed by a Patriot air defense system. Ukraine has received an unspecified number of the U.S.-made systems.

Mariana Bezuhla cited unnamed sources for her claim and demanded punishment for those responsible for the error.

Oleshchuk accused Bezuhla of defaming the air force and discrediting U.S. arms manufacturers, adding that he hoped she would face legal consequences for her claims.

The air force didn’t directly deny that the F-16 was hit by a Patriot missile.

The Institute for the Study of War said it was to be expected that Ukraine would lose some Western-provided military equipment in the fighting.

But the Washington-based think tank added that “any loss among Ukraine’s already limited allotment of Western-provided F-16s and trained pilots will have an outsized impact on Ukraine’s ability to operate F-16s in the near term as part of its combined air defense umbrella or in an air-to-ground support role.”

Overnight, Russia fired 18 Shahed drones and one ballistic missile at Ukraine, the air force said. It said 12 drones were destroyed, with four more going down before reaching their target.

The attack killed one woman and started a fire in Sumy, the capital of the region of the same name that borders Russia’s Kursk region, the regional administration said.

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