Grounded since May 3, the U.S. fleet of F-22 Raptors have cockpits that are filling with toxins entering through the pilot's life support system.

The Air Force Times cites "sources with extensive F-22 experience" for the story, but reports that despite three months of investigation, no one is sure what toxins caused the 14 F-22 incidents.

Each episode centers on pilots who suffer from "hypoxialike symptoms" and lose cognitive function and recall. One pilot couldn't change radio frequencies, another scraped tree tops on his final approach and was later unable to recall the incident.

Toxins found in pilots’ blood include oil fumes, residue from burned polyalphaolefin (PAO) anti-freeze, and, in one case, propane. Carbon monoxide, which leaves the blood quickly, is also suspected.

“These guys are getting tested for toxins and they’ve [gotten] toxins out of their bloodstreams,” the source said. “One of the guys was expelling propane.”

Sources requested anonymity for fear of reprisal, but this is a sharp change from the Air Force's official statement that carbon monoxide was entering the cockpits from pilots starting their planes in their hangars.

This despite the fact that incidents, and one crash, occurred well into into flights or even during a day's second mission -- long after the planes had left the hangars.