Officials investigating why a panel on a Boeing 737 Max 9 blew open during an Alaska Airlines flight last week say they are struggling to piece together exactly what happened because the plane’s cockpit voice recorder overwrote itself before it could be retrieved.

This is not a new problem. The National Transportation Safety Board, which is leading the investigation, has recommended for years that recorders be programmed to capture up to 25 hours of audio before automatically resetting themselves, but the Federal Aviation Administration has been reluctant to mandate longer recordings.

The F.A.A. last month proposed 25-hour recorders on new planes but argued that adding them to the existing fleet of U.S. planes would be too expensive. In addition, a pilots’ union has opposed the move to 25-hour recordings unless Congress puts in place protections that would prohibit their release to the public.

The chairwoman of the safety board, Jennifer Homendy, said the agency’s investigators had conducted 10 investigations since 2018 in which the cockpit voice recorder had been written over, with critical recordings lost forever. The voice recorders are among the key pieces of evidence that investigators use in reconstructing the events that led up to accidents as they work to establish a cause.

Ms. Homendy said a recording from the Alaska Airlines flight would have contained a lot of important information, including the bang that the crew described hearing soon after the plane took off on Friday from Portland, Ore. She said the recording would have enabled investigators to hear communications between the crew during the incident and identify any communications problems, including any audible alerts in the cockpit.

“There’s so much information that we can get off of C.V.R. that’s outside of just the communication amongst the flight crew,” Ms. Homendy said. “That is such a key piece of evidence to improve safety. Without that, we are piecing together things from interviews and losing a lot.”

Members of the flight crew told federal investigators that they had been so focused on going through their emergency checklist, communicating with air traffic control and getting the plane on the ground that they hadn’t heard any alerts. Federal investigators have not implied that the pilots or the flight’s crew made any errors.

“So now that’s what they don’t remember, and we have no evidence that it was happening,” Ms. Homendy said. “So if there was some sort of failure of any sort of oral alert, we wouldn’t know about it.”

Alaska Airlines said in a statement on Wednesday that because of the active investigation, it could not comment on why audio from the cockpit recorder was not recovered in time. But the airline added that it welcomed the F.A.A. proposal to lengthen the recording time.

“We support this effort, which would put the U.S. airline industry more in line with international regulations,” the airline said.

The United States had lagged behind much of the world in requiring the use of longer voice recordings in commercial planes. In 2016, the International Civil Aviation Organization, an arm of the United Nations, adopted a standard calling for recorders capable of capturing the last 25 hours of audio on all new aircraft starting in 2021. The European Union Aviation Safety Agency’s 25-hour mandate went into effect in January 2021 for new planes.

Cockpit voice recordings begin the moment pilots start a plane. This enables the recording to capture the pilots’ preflight checks, passenger boarding and other activities as the crew prepares for takeoff.

The two-hour limit means that the recorder may be quickly overwritten even on short flights, especially if there are any delays on the runway. Once the two-hour limit is reached, the recording automatically starts over.

Recorders are designed to automatically stop when there is an accident, but they do not stop in incidents like the one on Alaska Airlines’ 737 Max 9. In such cases, someone would have to remove a circuit breaker on the plane to prevent the device from starting over again. That did not happen in this case.

The safety board began to recommend increasing the recording time after a harrowing incident in 2017 at San Francisco International Airport when an Air Canada plane almost landed on a taxiway instead of a nearby runway. Four planes loaded with passengers were waiting on the taxiway. The incident could have been one of the worst aviation disasters in history, but federal investigators still have no idea what was going on in the cockpit because the recording automatically started over before it could be retrieved.

Robert Sumwalt, who was the chairman of the safety board at the time, said recordings from major aviation incidents could give federal investigators a more complete picture of what happened and how to prevent it from happening again.

“It gives you the pretty much firsthand account of what conversations and what sounds are in the cockpit,” he said. “People can think that they remember things clearly, but sometimes memory fails us.”

The F.A.A. in December proposed a rule that would require new planes to be equipped with 25-hour voice recorders but stopped short of mandating that commercial airlines install the recorders on all planes, as the N.T.S.B. has recommended.

The F.A.A. estimated that upgrading every plane would cost $741 million. Putting the new recorders only on new planes would cost $196 million.

“Our proposed rule aligns with regulations set by the International Civil Aviation Organization and European Union Aviation Safety Agency,” the agency said in a statement.

Ms. Homendy said saving lives should outweigh any financial concerns. She also pointed out that the lasting impact of a catastrophic plane crash would be far greater than the immediate cost of a safety upgrade that would be borne by airlines and, ultimately, travelers.

“The cost would be substantial, not just in terms of finances but in terms of the reputation of the company, in terms of the reputation of the manufacturer and suppliers and everyone else involved, and the cost of the public’s trust in the U.S. aviation system,” Ms. Homendy said. “That’s what would be lost immediately.”

Congress has also taken note of the issue. Bills pending in the House and Senate to reauthorize the F.A.A. would extend the duration of the recording to 25 hours on all planes within four years.

Since the incident in San Francisco in 2017, Representative Mark DeSaulnier, a California Democrat on the House Transportation and Infrastructure Committee, said he supported the safety board’s recommendation on voice recorders because critical data was often lost because investigators couldn’t retrieve it quickly enough.

“Moving to 25-hour cockpit voice recorders is an essential component of advancing air travel safety that has already been adopted as the international standard,” Mr. DeSaulnier said.

But the Air Line Pilots Association, which represents pilots at Alaska, Delta Air Lines, United Airlines and other companies, has long opposed the move to a 25-hour voice recorder citing privacy concerns. In a statement, the union said that while voice and flight data recorders provided critical information, the group wanted lawmakers to make sure that investigators used the recordings only to improve the aviation system.

Federal law prohibits the safety board from releasing copies of cockpit voice recorders under freedom-of-information laws. But the law does not prevent the F.A.A. or airlines from releasing copies.

“Unfortunately, the legal statute that protects the privacy of the cockpit voice recorder only applies to N.T.S.B.,” the statement said. “In addition to the N.T.S.B., the protections in that statute need to be strengthened and applied to the airlines as well as the F.A.A. before considering expanding the duration.”

Senator Ted Cruz, a Texas Republican, said that despite the pushback from the pilots’ union, he and other members of Congress planned to advance legislation to increase the recording time.

“Without access to cockpit voice recordings, investigators lack essential information about any troubling incident, whether it’s a near miss, an equipment failure or the recent Alaska Airlines flight,” Mr. Cruz said in an interview.

Niraj Chokshi contributed reporting.