By Richard A. Oppel Jr. and Thomas Fuller

Whenever there is a plane or major highway crash, federal investigators descend immediately to the scene to collect evidence. Their goal is to find the accident’s cause so that future tragedies can be prevented. 

But during the 35-day government shutdown, the National Transportation Safety Board furloughed all but a handful of its 400 staff members. And investigations into at least 18 fatal accidents in the United States that normally would have meant sending experts to the scene were not begun, according to the agency.

Only now are investigators who returned to work this week beginning to examine the accidents, which led to at least 32 deaths. And some experts fear that crucial evidence has been lost.

“Trying to go back after the fact and reconstruct all the facts, elements and circumstances will be very difficult, or impossible,” said Greg Feith, a former senior air safety investigator at the agency who is now a private consultant.

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.

Those included the October crash of Lion Air Flight 610, a Boeing 737 that plunged into the Java Sea shortly after takeoff, killing all 189 on board.

“Who made the decision that the board is going to perform some of its functions, but not others?” asked Jim Hall, who was chairman of the safety board during the 1990s, when he said he kept investigators working during government shutdowns.

Mr. Hall added, “The fact of the matter is that if you are not able to respond, you have weakened the investigation.”

Just how much has been lost is not clear. The safety board declined to characterize which investigations had been most impaired by passage of time and spoilage of evidence. 

A spokesman, Christopher O’Neil, said the agency “closely followed” guidance from the federal Office of Personnel Management during the shutdown.

Under the law, Mr. O’Neil said in an email, staff members “could only be recalled to investigate an accident if their work was necessary to prevent imminent loss of life or significant property damage.”

He said agency officials assessed every accident reported during the shutdown. Only two — the failure in late December of a United States-manufactured engine on a South Korean airliner, and the continuing inquiry into the Lion Air crash — met the threshold for recalling furloughed investigators.

But the agency concedes that investigators may never visit some accident sites, and opportunities to learn things that could prevent tragedies have been missed.

Seven people died in a car accident near Gainesville, Fla., this month. Federal accident investigators were not sent to the crash site because of the shutdown.

“Important evidence was lost that we would normally examine following an accident,” Mr. O’Neil said in a statement. That, he added, “potentially could prevent determination of probable cause.”

Another accident not investigated during the shutdown was a fiery multivehicle highway crash in Florida on Jan. 3 that killed five children headed to Walt Disney World, and two others. An eight-member team is being dispatched, the agency said on Wednesday.

In addition, the agency did not gather evidence needed to determine whether investigations were warranted in five other highway, railroad and pipeline accidents that left eight more people dead.

Federal investigators returning to their jobs in many cases will have to rely on information collected by local law enforcement, whose evidence gathering may not be as precise or granular.

And a lot is simply lost, too, by not being able to see the disaster firsthand, and how wreckage was strewn about.

Examples of important but highly perishable evidence include the distance between propeller marks in the snow or mud — which help estimate an aircraft’s speed at impact — or chunks of ice shaped like a leading edge, which tell investigators that ice may have been on a wing.

By now, Mr. Feith said, at many crash sites the wreckage will have been hoisted onto trucks, often after being cut into pieces, and shipped to storage facilities.

There, agency investigators will sift through the pieces, not always knowing which fractures and fatigue happened before the crash, which were caused by the crash, and which were caused by moving the wreckage afterward.

“They may not be able to decipher what is an artifact of the accident, versus what is an artifact of the post-accident recovery,” Mr. Feith said.

Moreover, another key evidence collection — requests for air traffic control data normally made quickly after a crash — may not be possible in some cases. “A lot of that information is lost in the abyss,” he said.

The significance of getting investigators to accident sites speedily is underscored by how the safety agency organizes its inquiries: A “Go Team” of rotating experts fly immediately to the most significant crash sites. They are on call 24 hours a day, with a bag of audio recorders, cameras, flashlights, screwdrivers, wrenches or other tools at the ready.

“The importance of being on site as soon as possible is why the agency is structured the way it is,” said Mr. Hall, the former agency chairman.

It is not only the physical evidence that fades or breaks down, he added.

“Individuals’ memories further away from the event can be less than certain than what might have been, had they been interviewed as soon as the event occurred,” he said.

The shutdown also may have slowed some investigations by the Federal Aviation Administration — including an inquiry into a Jan. 21 near miss above a heavily populated neighborhood of Oakland, Calif.

During the mishap, which has not been previously reported but was confirmed by the F.A.A., two single-engine planes came within 100 to 200 feet of each other, the two pilots said in interviews. The Oakland control tower mistakenly instructed them to fly at the same altitude even as they flew toward each other, the pilots said.

“I was shocked by how close he was,” one pilot, Isaac Reynolds, said of the other aircraft. “We very easily could have hit each other.”

An F.A.A. spokesman, Ian Gregor, said the Oakland control tower filed a report about the near miss on the day it occurred, and that it was investigated the next day.

But Mr. Reynolds said he was never contacted. And the other pilot, who requested to remain anonymous because he did not want his family to know about the near miss, said when he called the airport to report the incident after landing, he was told the investigation would be slower because of the shutdown.

“They said things were slower than usual because the person who would investigate was on furlough,” the pilot said. “I’m not trying to throw anyone under the bus, but I do think that shutdowns matter. Whether or not this contributed or not it’s hard to say, specifically. But it’s certainly not helping safety.”