U.S. authorities gave green light this week for the $143-million F-22 Raptors to resume flight training missions despite ongoing investigation on the combat plane's alleged mid-flight glitches that leave its pilots gasping for air during flights.

Officials of the United States Air Force have indicated that initial probe suggested that aviators flying the fifth-generation stealth aircraft suffered symptoms similar to 'Hypoxia', a condition that occurs when a person's brain is not getting sufficient oxygen supply.

Air Force investigators, according to ABC News, have determined that the dizziness reported by F-22 Raptors pilots was closely related to hypoxia, which they explained could actually induce confusion and poor judgement during flight missions.

The latest findings, however, failed to establish the real cause of the problem, according to U.S. Air Force spokesperson Lt. Col. John Haynes, who stressed that "it's not just that 'the problem' wasn't identified ... there was no conclusive cause or group of causes."

"In different situations, there were different types of symptoms at different times. There was no common thread they (investigators) found to link all these together," the Air Force official further explained as reported by ABC.

Prior to its return to air Tuesday this week, U.S. officials have decided to ground the Air Force's entire fleet of 189 F-22 Raptors for more than four months earlier this year owing to previous incidents that saw pilots on the new combat plane struggling to breathe, which were later adjudged as similar to what occurred last week.

Amidst concerns that pilot safety may be compromised by the continued use of the Raptors, Haynes clarified that the U.S Air Force is observing extreme measures to ensure that no disaster will attend the stealth fighter's training missions, which he noted were being conducted on U.S. Air Force facilities across the globe.

To date, one fatality has been connected to F-22 mid-flight problems with the death in November last year of Capt. Jeffrey Haney, who according to ABC was killed in a crash while piloting the controversial aircraft during a night-time mission.

Also, the F-22s, which U.S. Air Force officials have envisioned as the best aircraft available for America to maintain air superiority over its nearest rivals, are not without its share of political snags with the U.S. Congress blocking in 2009 the Pentagon's plan of purchasing some 600 Raptors from Lockheed Martin.

The deal would have amounted to more than $77 billion but U.S. lawmakers have argued that current challenges facing America gear towards more conventional foes as oppose to the sophisticated air skirmishes that the F-22 is exclusively designed for.