According to CNN, the US Navy has linked the deaths of four F/A-18 Hornet pilots to “physiological episodes” that may be tied to oxygen contamination or other human factors that occurred while the aircraft was airborne, according to an in-depth review of oxygen system failures.

Released on Thursday, the Navy report details findings of an investigation that was launched in March after more than 100 T-45 instructional pilots refused to fly in protest at continuing issues with the aircraft’s oxygen system.

That protest occurred just days after Navy officials described a rising rate of “physiological episodes” (PEs) affecting pilots who fly all models of F-18 aircraft, especially the Boeing-built Super Hornet, during testimony on Capitol Hill.

Pilots reported that physiological episodes resulting in dizzyness or even blackouts have been caused by oxygen contamination, human factors — including air sickness and vertigo, failure of the on-board oxygen generation system and the failure of other key systems — according to March testimony from Rear Adm. Michael Moran.

According to the Navy, the four F-18 pilot deaths, which occurred over a span of 10 years, are not all the direct result an oxygen system failure but are linked by the fact that pilots experienced various symptoms that fall within the scope of what is described as a physiological episode.