The pricey F-22 Raptor jet has just gotten back up in the air, but the safety problem that grounded it doesn't seem to be resolved.

Last year, the F-22 was grounded for four months because pilots were experiencing dizziness and other symptoms of hypoxia, which is caused by a lack of oxygen. The Air Force looked into possible malfunctions in the plane’s oxygen-generation system, but in September, the planes were cleared for service after technicians were unable to pinpoint a source of the problem.

Yesterday, however, the Air Force’s Air Combat Command confirmed that some pilots — they would specify only “a very small” number — have requested not to fly the F-22.

General Mike Hostage, who heads the Air Combat Command, said in a news briefing yesterday that the Air Force is taking cautionary measures but would continue to fly the planes. “We don’t have a conclusive answer yet, and that’s why we continue to fly with the mitigating procedures, because I can’t learn about the problem if I don’t fly the airplane,” he said.

Since the planes started flying again in September, there have been more than 12,000 sorties and 11 reported instances of “hypoxia-like symptoms.” An Air Combat Command Center spokesman told ProPublica today that a team of two-dozen Air Force and outside specialists is monitoring the planes and pilots for both mechanical and medical problems regarding the hypoxia symptoms, but that no “root cause” has been determined.

Before the grounding, there had been at least 12 separate reports of hypoxia-like symptoms, and planes had been limited to flying at lower altitudes. In late 2010, an F-22 pilot died in a crash after he apparently lost control of the plane when the oxygen system malfunctioned. The Air Force’s official report on the incident acknowledged the oxygen system failure but blamed the pilot’s response for the crash.

As ProPublica has detailed, the roughly $70 billion F-22 program has long experienced structural deficiencies and cost overruns. The U.S. halted orders of the jets in 2009, as then-Secretary of Defense Robert Gates argued the F-22’s specific capability was not widely applicable in the nation’s “spectrum of conflict.”

The planes have yet to be deployed in combat, though last week a number of them were reportedly sent to the United Arab Emirates.