Senior Air Force leaders still have no idea why some pilots of $415 million F-22 fighters, the most expensive warplanes ever, keep blacking out mid-flight.

The service has concluded a yearlong investigation into a rash of incidents, including one death, in which pilots reported difficulties breathing due to apparent problems with the onboard oxygen system. But that probe failed to produce anything definitive on the causes.

"There is lingering uncertainty that the Scientific Advisory Board found no root causes," Air Force Secretary Michael Donley told reporters at a Thursday breakfast meeting.

The entire 188-plane fleet was grounded for months, but Donley says he was satisfied enough with the oxygen system study to clear all the jets to resume flights.

"This is still a relatively rare event," Donley says. "This is not common."

After countless design and testing ailments delayed delivery of the F-22 to the Air Force and drove up its price tag, then-Defense Secretary Robert Gates in 2009 capped the fleet at 188 jets. The Government Accountability Office pegs the cost of each F-22 at nearly $415 million; early estimates were around $140 million.

Despite being declared ready for combat in 2005, Washington has never sent the Raptors into harm's way. Many lawmakers and defense analysts questioned why the highly stealthy jet was not used to evade rudimentary Libyan air defenses last year. Air Force Chief of Staff Gen. Norton Schwartz last February signaled the Raptor would be used in Libya; weeks later, however, he said it was not used because it was based too far away.

The air service intends to continue collecting information and monitoring the oxygen system, has made some changes to the fleet, and put in place new emergency procedures, Donley says.

Pilot blackouts and dizzy spells have occurred in one out of every 10,000 flight hours, according to the Air Force. (In 2011, the Air Force revealed 14 cases of a condition known as hypoxia, which can trigger severe headache, nausea and fatigue when the body gets too little oxygen.)

Some Pentagon observers are blasting the Air Force for failing to ensure its pilots will avoid the same fate as Capt. Jeffrey Haney, who died when his Lockheed Martin-made F-22 crashed in Alaska. His widow is suing Lockheed and other firms that did work on the jet.

"It's disturbing that the Air Force would put its pilots at risk of suffocation when the Air Force still doesn't know the root cause of the problem even after extensive study," says Ben Freeman, national security investigator for the Project on Government Oversight. "Congress needs to conduct oversight on why this problem exists at all at this stage in a program that's decades old."

One former congressional defense aide says if the service cannot ensure the F-22 is not a killer jet, heads should roll.

"They remain clueless as to the cause," says longtime military analyst Winslow Wheeler, now with the Center for Defense Information. "They can't even support [a] frequency prediction. ... They have failed to give their pilots a reliable, safe breathing apparatus."

Wheeler slammed the service for "literally telling the pilots to 'suck it up.'"

Wheeler also raises an intriguing question: What would have happened if Gates had been confronted with an Air Force finding that amounts to a leadership shoulder shrug? Gates was known for his sharp bureaucratic elbows and willingness to fire service leaders and managers of failing weapon programs.