WASHINGTON — Regional airlines report difficulty over the past year in finding enough pilots to hire, but it isn’t clear whether there is a shortage of pilots.

Researchers found “mixed evidence” of a shortage, said the Government Accountability Office. A key economic indicator supports the emergence of a shortage, but two other indicators suggest the opposite is true. Of three studies reviewed by the GAO that examine the issue, “two point to the large number of qualified pilots that exist, but may be working abroad, in the military, or in another occupation, as evidence that there is adequate supply,” the report said.

The report, obtained late Thursday by The Associated Press, was to be released Friday.

The U.S. airline industry will need to hire 1,900 to 4,500 new pilots annually over the next 10 years due to an expected surge in retirements of pilots reaching age 65 and increased demand for air travel, the report said.

Eleven out of 12 regional airlines failed to meet their hiring targets for entry-level pilots last year, the report said. However, no major airlines were experiencing problems finding pilots. Some airline officials did express concern that service to some cities might have to be cut back if their regional partners cannot hire enough pilots, the report said.

Major airlines generally pay significantly higher salaries than regional carriers. The average starting salary for first officers at regional airlines is $22,400, according to the Air Line Pilots Association.

“Data indicate that a large pool of qualified pilots exists relative to the projected demand, but whether such pilots are willing or available to work at wages being offered is unknown,” the report said. And, the size of these pools of pilots has remained steady since 2000, the report said.

There are currently 66,000 pilots working for U.S. airlines, but there are 109,465 currently active pilots with a first class medical certificate that are licensed to fly passengers, the report said. Plus, there are more than 100,000 other pilots with commercial licenses that might at some point choose to pursue an airline career, the report found.

The unemployment rate for professional pilots is very low, only 2.7 percent. That would normally indicate a shortage, but there are other reasons to believe that may not be the case, according to the report. Average professional pilot salaries went down 9.5 percent from 2000 to 2012, while the number of pilots employed went up 12 percent. Both trends are inconsistent with a shortage, the report said.

Airlines used to be able to hire first officers with as few as 250 hours of flying experience. New regulations that went into effect last year require that all airline pilots now have a minimum of 1,500 hours. Previously, only captains were required to have that much experience.

Major carriers typically hire both captains and first officers with far more than 1,500 hours. But regional carriers often drop well below that when hiring entry-level first officers.

The new regulations were required under an aviation safety law Congress passed more than three years ago in response to a 2009 crash of a regional airliner near Buffalo, N.Y., that was blamed on pilot error. All 49 people on board and a man on the ground were killed.

A subsequent investigation revealed that the first officer had been paid only about $16,000 the previous year, which was her first year at the airline. The captain was earning about $63,000. The Continental Connection flight was operated by the now-defunct regional carrier Colgan Air Inc.

The National Transportation Safety Board concluded that both pilots were suffering from fatigue, although the board stopped short of citing fatigue as a contributor to the crash. Neither pilot had sleep in a bed the night prior to the fatal flight.

The captain appeared to have tried to nap in a pilot lounge where sleeping was discouraged. The first officer, who lived at home with her parents, commuted across the country over night in a jump seat in order to make the fatal flight.

In congressional hearings afterward, some lawmakers questioned whether the pilots couldn’t afford hotel rooms on their salaries.