The Navy Department halted Wednesday its entire fleet of T-45 trainer jets after instructor pilots raised concerns about the ability of aviators to breathe in them while flying, military officials said, expanding the scope of safety concerns about U.S. military aircraft.

Senior Navy officials made the decision amid problems with “physiological episodes,” said Cmdr. Jeanette Groeneveld, a Navy spokeswoman. The term defines incidents in which the amount of oxygen a pilot breathes is reduced or contaminated or a cockpit is depressurized, causing lightheadedness and, in the worst cases, blackouts.

Rear Adm. Dell Bull, the chief of naval air training, will keep all 197 of the single-engine, black-and-orange aircraft on the ground as engineering experts meet in person with pilots at training wings in Kingsville, Tex.; Meridian, Miss.; and Pensacola, Fla., Groeneveld said. The meetings began Monday in Kingsville, and will continue through Friday.

“We take the concerns of our air crew seriously and have directed a two-day safety pause for the T-45 community to allow time for naval aviation leadership to engage with the pilots, hear their concerns and discuss the risk mitigations, as well as the efforts that are ongoing to correct this issue,” Groeneveld said.

The “operational pause” could be extended longer depending on what is discovered, she said. The issue has primarily been caused by contamination in breathing systems. The halt is not technically considered a grounding of the aircraft because nothing specific has been identified to fix, but one could be required as more is learned.

“There has not been a smoking gun,” Groeneveld said. “There is nothing that has said, ‘This is it,’ and we can fix it and move on.”

Fox News reported Monday night that more than 100 Navy pilots were boycotting flying the T-45 until the issue is resolved. One of the pilots potentially affected is Marine 1st Lt. Michael Pence, the son of Vice President Pence, the report said. He is a flight student at Naval Air Station Meridian in Mississippi.

Groeneveld said she would not characterize the decisions not to fly as a boycott, but acknowledged that senior Navy officials have decided to meet personally with some instructor pilots. There are typically more than 200 training flights per day, she said.

The grounding of the T-45s comes a week after senior U.S. military officials told the House Armed Services Committee’s subcommittee on tactical air and land forces that the F/A-18 Hornet, F/A-18 Super Hornet, EA-18 Growler and T-45 all have seen a sharp spike in physiological episodes since 2010, when the Navy Department began tracking the issue.

The worst spike occurred in the Growlers, which carry out electronic warfare. There were 5.52 physiological episodes recorded per 100,000 hours of flight in the plane in fiscal 2011, but the rate jumped to 15 per 100,000 hours in fiscal 2014, 42.9 in fiscal 2015 and 90.8 in fiscal 2016, according to congressional testimony.

The Hornet also saw significant problems. In fiscal 2011, there were about 10.9 physiological episodes per 100,000 hours of flight, but the rate jumped to 21 in fiscal 2013 and 57.2 in fiscal 2016.

The T-45 had about 11.9 physiological episodes per 100,000 hours of flight in 2012, but the rate jumped to 18.4 by 2014 and about 47 in 2016, according to Navy statistics.

A Navy officer, speaking on the condition of anonymity because of the sensitivity of the situation, said the problem in the T-45 is likely even worse recently. He characterized the decision of flight instructors to not fly as “not so much a strike,” but instead “instructors invoking their responsibility to not fly when there is significant enough risk to the aircraft and personnel.”

Navy instructors decided not to fly Monday, while Marine Corps pilots continued to fly, the officer said. The admiral’s decision to ground the entire T-45 fleet affects all of them, he added.

Vice Adm. Mike Shoemaker, the commander of all naval air forces, has sent at least three messages to air crews since 2015 about problems with naval aircraft and encouraged service members to report them, military officials said.

“We are working diligently to determine all the root causes of [physiological episodes] and find solutions, and we will continue to aggressively prioritize resources and engineering efforts,” Shoemaker said in the most recent one, sent in January. “Alert aircrew remains our best line of defense… Your awareness and immediate actions at the first signs of something ‘not quite right’ are critical, leading to safe aircraft recoveries and the identification of key causal factors.”

Thomas Gibbons-Neff contributed to this report.