Most of the accidents were crashes of small planes, like one that plunged into Chickamauga Lake, Tenn., on Jan. 7, killing the pilot, Frank Davey, and sole passenger, Lynda Marinello.

Ms. Marinello’s husband, Chris, a pilot for 20 years, said he feared that once the safety board was finally able to examine the wreckage, it would default to a finding of “pilot error” because evidence supporting other causes might have been spoiled with no one from the agency there to safeguard it.

In an interview, Mr. Marinello said that Mr. Davey was a good pilot, and that three cameras were recovered from the wreckage that could yield clear evidence of what brought the plane down. But he worries the information on the cameras may be badly degraded.

“If the N.T.S.B. guys would have been on the scene, they would have understood the importance of getting those SIM cards to Washington or to some facility that had the ability to get the data,” he said. He says he has asked but has not been told where the cards are, or whether they are locked with other wreckage in storage.

While safety board members are appointed by the president, it is an independent federal agency. The potential impairment of so many investigations has prompted some air-safety experts to question why the agency did not keep more investigators working in the United States, even as some were taken off furlough to help with crashes in other countries.