WASHINGTON >> The Air Force believes it is getting much closer to pinpointing the reason why pilots of its prized F-22 stealth fighters sometimes suffer an oxygen deficit during flight, a senior general said Thursday.

The problem prompted the Air Force to ground the aircraft for a period in 2011, and Defense Secretary Leon Panetta last month ordered new flight restrictions after CBS’ “60 Minutes” program aired interviews with F-22 pilots who complained about the oxygen problem.

Maj. Gen. Charles Lyon, who is managing the Air Force’s effort to resolve the mysterious problem, said in an Associated Press interview that it is likely that the pilots’ symptoms are caused by previously unknown restrictions on their breathing.

“We’re not ready to declare victory yet,” he said, but this is the first time the Air Force has narrowed down the likely cause. Lyon said he also is close to ruling out another theory: That contaminants were getting into the pilot’s lungs via the oxygen delivery system that is connected by hose to their flight helmets.

Lyon said he is satisfied, after extensive testing, that no harmful contaminants are moving through the oxygen system. He is the director of operations for Air Force Combat Command and has been leading the F-22 work since January.

Lyon said the root of the problem, which has caused some F-22 pilots to feel dizzy and experience other symptoms of hypoxia, or oxygen deprivation, may turn out to be linked to two issues:

—Improper functioning of the pilots’ pressure, or G-force, vest. Lyon said that, unknown to the pilots, the vest’s bladder has been filling with air at times when it should not. That has made it harder for the pilots to breath. The Air Force last Friday stopped using the vests and is going to modify them before returning them to use in the F-22, Lyon said. In the meantime the Air Force has lowered the maximum altitude the F-22 will fly, since the vests are intended to protect pilots’ lungs in the event of a sudden loss of cockpit air pressure at high altitudes.

—The hose and hose connectors that are part of the pilot’s oxygen delivery system have been leaking slightly, further restricting the amount of oxygen getting to the pilot’s lungs.

Lyon said additional testing will be done before the Air Force can be certain that these restrictions are the root of the problem.

The Air Force has come under fire from some in Congress for not taking quicker action to fix the problem. Two leading critics, Sen. Mark Warner, D-Va., and Rep. Adam Kinzinger, R-Ill., told reporters Thursday they believe the Air Force needs to be more open with the public about the issue.

Warner and Kinzinger also said that information they received from the Air Force this week indicates that the oxygen-deficit problem is greater than the Air Force had previously believed.

In the AP interview, Lyon acknowledged that there had been a higher rate of hypoxia-like incidents among F-22 pilots since the plane was returned to flight in September than in years past. He said there were 11 such incidents from September to March 8. But since then, over a period in which the F-22 has flown about 6,000 hours, there have been none, Lyon said.

“The trend line is very positive,” the general said.

Lyon said he briefed senior Pentagon officials and congressional staff members this week, including the staffs of Warner and Kinzinger, on all aspects of his work to solve the F-22 problem.

The F-22, manufactured by Lockheed Martin, is the Air Force’s most-prized stealth fighter. It was built to evade radar and is capable of flying at faster-than-sound speeds without using afterburners.

The 170-jet fleet is stationed at six U.S. bases: Joint Base Elmendorf-Richardson, Alaska: Joint Base Pearl Harbor-Hickam, Hawaii; Joint Base Langley-Eustis, Va.; Nellis Air Force Base, Nev.; Holloman Air Force Base, N.M.; and Tyndall Air Force Base, Fla.  

F-22 pilots are trained at Tyndall. Flight testing is at Edwards Air force Base, Calif., and operational testing and tactics development is performed at Nellis.