Recent incidents with the Air Force’s fleet of F-22 Raptor fighter jets have prompted fresh questions by two members of Congress looking for answers on the oxygen problems that have plagued the aircraft for years.

F-22 pilots have reported dozens of incidents in which the jet's systems weren't feeding them a proper amount of oxygen, causing hypoxia-like symptoms in the air. Hypoxia is a condition that can cause nausea, headaches, fatigue or even blackouts.

On Tuesday, Rep. Adam Kinzinger (R-Ill.) and Sen. Mark R. Warner (D-Va.) held a teleconference with reporters, saying they wanted more information about three recent events.

The first was an incident on July 6 involving an F-22 pilot at Joint Base Pearl Harbor Hickam in Hawaii; the second was a “restricted airflow” incident in late June involving an F-22 pilot at Joint Base Langley-Eustis in Virginia; and the third involved a mishap at Tyndall Air Force Base in Florida on May 31 in which an F-22 reportedly hit the runway without extending its landing gear.    

“This seems to be a never-ending saga,” Warner said.

The congressmen want to know if the new incidents have anything to do with the ongoing oxygen issues that led to the grounding of the entire F-22 fleet last year for nearly five months.

Since the F-22 grounding was lifted last September, the Air Force said, there have been at least 11 incidents in which F-22 pilots reported hypoxia-like symptoms. The Air Force said investigators could not find a "smoking gun."

F-22 flights have been restricted by Defense Secretary Leon Panetta. Still, Air Force officials maintain that F-22s are safe to fly. New precautions for pilots have been put into place, such as wearing a device that measures the amount of oxygen in the blood, taking blood samples and watching over pulmonary functions.

Air Force officials believed the problems could be traced back to a pressure vest worn by pilots in the cockpit. But Kinzinger, a former Air National Guard pilot, said the new events prove that hypothesis to be flawed.

“I thought we were onto something,” he said.

The congressmen wrote a letter on Tuesday to Air Force Secretary Michael B. Donley in which they asked four pointed questions and several follow-ups regarding the hypoxia-like events and the ongoing investigation.

"We would ask that you provide us with a follow-up briefing on these issues within 30 days or sooner so that we might continue to work with you to ensure that the F-22 remains a vital component of theUSAF mission," they write.

As The Times has reported: The F-22, designed and built by  Lockheed Martin Corp., is considered the most advanced fighter jet in the world. It entered service in 2005, and the Air Force received the last of its order of 188 planes earlier this year.

The plane can reach supersonic speeds without using afterburners, enabling it to fly faster and farther. It's also packed with cutting-edge radar and sensors, enabling a pilot to identify, track and shoot an enemy aircraft before that craft can detect the F-22. The Air Force says the aircraft is essential to maintain air dominance around the world.

According to the Air Force, each of the sleek, diamond-winged aircraft costs $143 million. Counting upgrades and research and development costs, the U.S. Government Accountability Office estimates that each F-22 cost U.S. taxpayers $412 million.

While other warplanes in the U.S. arsenal have been used to pummel targets in Iraq, Afghanistan and Libya, the Air Force's F-22s have sat largely idle — used only in test missions.

Even so, throughout the jet's development, F-22 pilots have experienced seven serious crashes, including two fatalities.