The commander of Ukraine’s air force said on Friday that it had shared with U.S. partners a preliminary report about an F-16 jet crash that took place on Monday.

Mykola Oleshchuk said on Telegram that partners from the United States, where the F-16 is manufactured, were assisting the investigation.

The Ukrainian military said on Thursday that the F-16 crashed and its pilot died while approaching a target during a major Russian air strike on Monday.

Ukraine says US-made F-16 fighter jet crashed, killing pilot

A U.S. defense official told Reuters that Monday’s crash did not appear to be the result of Russian fire, and possible causes from pilot error to mechanical failure were still being investigated.

Deputy Pentagon press secretary Sabrina Singh said on Thursday that the U.S. had not been asked to participate in any type of investigation to look into the F-16 incident.

The arrival of the first F-16 jets was a milestone for Ukraine in the fight against the full-scale invasion Russia launched 2-1/2 years ago.