For the casual student of U.F.O. history, the modern idea of life beyond our planet usually dates to 1947, when a top-secret U.S. military balloon crashed in the desert near Roswell, N.M. The wreckage prompted decades of conspiracy theories and gave rise to the idea that Roswell was the site of an alien crash landing.

Now, thanks to a new congressional spending bill, U.F.O. enthusiasts may look to 1945 as the beginning of that era.

An amendment tucked into this year’s $858 billion National Defense Authorization Act, which funds the Defense Department’s annual operating budget, requires the department to review historical documents related to unidentified aerial phenomena — government lingo for U.F.O.s — dating to 1945. That is the year that, according to one account, a large, avocado-shaped object struck a communication tower in a patch of New Mexico desert now known as the Trinity Site, where the world’s first atomic bomb was detonated that July.

Experts said the bill, which President Biden signed into law in December, could be a game changer for studying unidentified phenomena.

“The American public can reasonably expect to get some answers to questions that have been burning in the minds of millions of Americans for many years,” said Christopher Mellon, a former deputy assistant secretary of defense for intelligence. “If nothing else, this should either clear up something that’s been a cloud hanging over the Air Force and Department of Defense for decades or it might lead in another direction, which could be truly incredible. There’s a lot at stake.”

The amendment was introduced by Representative Mike Gallagher, a Wisconsin Republican and a member of the Armed Services Committee. Mr. Gallagher, who declined an interview request, said in a brief statement that a “comprehensive timeline” of unidentified aerial phenomena in U.S. government records was needed, and that the amendment would ensure a full review of “all U.S. government classified and unclassified information.”

“This is an important step that will give us a more comprehensive understanding of what we know — and don’t know — about incidents impacting our military,” he said.

The U.S. government has dabbled in public-facing programs that have explored the possibility of alien life. In 2021, the Pentagon announced it would form a task force to look at the issue after a congressionally mandated report found that the government had no explanation for 143 sightings of strange phenomena by military pilots and others since 2004. NASA said in June that it would finance a study to look at unexplained sightings.

In 2022, the Defense Department established the All-domain Anomaly Resolution Office, which succeeded the Airborne Object Identification and Management Synchronization Group, after facing scrutiny from the public and lawmakers. Sean M. Kirkpatrick, a former chief scientist at the Missile and Space Intelligence Center, which is part of the Defense Intelligence Agency, was named director.

The introduction of drones and other airborne clutter has led to an increase in sightings of unidentified objects in recent years. The government, which delivered its most recent update on Thursday, has found that weather balloons and surveillance operations by foreign powers accounted for most of the recent sightings of unidentified aerial phenomena, though dozens remain officially unexplained.

The defense funding bill requires the new office to work with the intelligence community to identify any nondisclosure agreements related to possible U.F.O. sightings. It also requires the office to create a process for people to share information, regardless of classification, and to share its findings with the highest levels of the Defense Department. It also mandates that the All-domain Anomaly Resolution Office be fully staffed.

“This is an office now that has a voice and resources, and it has authority,” Mr. Mellon, the former Defense Department official, said.

Susan Gough, a spokeswoman for the agency, said in an email that “the department is reviewing the enacted legislation.”

Jacques Vallée, a longtime ufologist, astronomer and computer scientist, said the amendment’s inclusion in the defense bill was “an absolute turning point.”

“This is what all scientists and my colleagues have always dreamed of,” said Dr. Vallée, who has helped study reports of U.F.O.s for the Centre National d’Études Spatiales, the French space agency. He said that the U.S. government’s agreement to dig into the past meant “the stigma has been removed.”

Dr. Vallée began studying the Trinity incident several years ago alongside a journalist, Paola Harris, and interviewed people who claimed to have witnessed the crash. Dr. Vallée and Ms. Harris chronicled their research in a book, “Trinity: The Best-Kept Secret,” including the details of the avocado-shaped object. They also spoke to witnesses who said they came across the object as children and found what they described as “little creatures.”

In the United States, Dr. Vallée said, “there has always been, on the part of the government, especially the Pentagon,” a sense that civilian sightings are unreliable. “The reason,” he said, “is that civilians don’t have the technology to really document what happens, and of course the Pentagon does.”

But, Dr. Vallée said, there’s no reason that “a farmer in his field” isn’t qualified to give a quality observation of a possible U.F.O. “The civilian observations tend to be longer, they tend to be more detailed, they tend to leave a trace that we can analyze,” he said.

He said he was working with a team at Stanford University to analyze samples of minerals and debris that were left after U.F.O. crashes or landings.

“I would hope that the new project would continue to do that because I think we’ve shown the way to do that scientifically,” Dr. Vallée said. He added, “We don’t have proof that a biologist can look at, but we have considerable statistical and now observational evidence that there must be life out there, that the Earth is not unique.”

At 83, Dr. Vallée still holds out hope for tangible evidence in his lifetime.

“Science is a moving frontier,” he said. “I want to have the right answers, even if they are small answers, rather than more speculation.”