The Federal Aviation Administration is investigating claims made by a Boeing engineer who says that sections of the fuselage of the 787 Dreamliner are improperly fastened together and could break apart mid-flight after thousands of trips.

The engineer, Sam Salehpour, who worked on the plane, detailed his allegations in interviews with The New York Times and in documents sent to the F.A.A. A spokesman for the agency confirmed that it was investigating the allegations but declined to comment on them.

Mr. Salehpour, who has worked at Boeing for more than a decade, said the problems stemmed from changes in how the enormous sections were fitted and fastened together in the assembly line. The plane’s fuselage comes in several pieces, all from different manufacturers, and they are not exactly the same shape where they fit together, he said.

Boeing conceded those manufacturing changes were made, but a spokesman for the company, Paul Lewis, said there was “no impact on durability or safe longevity of the airframe.”

Mr. Lewis said Boeing had done extensive testing on the Dreamliner and “determined that this is not an immediate safety of flight issue.”

“Our engineers are completing complex analysis to determine if there may be a long-term fatigue concern for the fleet in any area of the airplane,” Mr. Lewis said. “This would not become an issue for the in-service fleet for many years to come, if ever, and we are not rushing the team so that we can ensure that analysis is comprehensive.”

In a subsequent statement, Boeing said it was “fully confident in the 787 Dreamliner,” adding that “these claims about the structural integrity of the 787 are inaccurate and do not represent the comprehensive work Boeing has done to ensure the quality and long-term safety of the aircraft.”

Mr. Salehpour’s allegations add another element to the intense scrutiny that Boeing has been facing since a door panel blew off a 737 Max jet during an Alaska Airlines flight in early January, raising questions about the company’s manufacturing practices. Since then, the plane maker has announced a leadership overhaul, and the Justice Department has begun a criminal investigation.

Mr. Salehpour’s concerns are set to receive an airing on Capitol Hill. Senator Richard Blumenthal, Democrat of Connecticut and the chairman of the Senate Homeland Security and Governmental Affairs Committee’s investigations subcommittee, is planning to hold a hearing with Mr. Salehpour on April 17. Mr. Blumenthal said he wanted the public to hear from the engineer firsthand.

“Repeated, shocking allegations about Boeing’s manufacturing failings point to an appalling absence of safety culture and practices — where profit is prioritized over everything else,” Mr. Blumenthal said in a statement.

The Dreamliner is a wide-body jet that is more fuel efficient than many other aircraft used for long trips, in part because of its lightweight composite construction. First delivered in 2011, the twin-aisle plane has both racked up orders for Boeing and created headaches for the company.

For years, the plane maker has dealt with a succession of issues involving the jet, including battery problems that led to the temporary grounding of 787s around the world and quality concerns that more recently caused an extended halt in deliveries.

Boeing has also confronted a slew of problems at its plant in South Carolina where the Dreamliner is built. A prominent Boeing whistle-blower who raised concerns about manufacturing practices at the plant, John Barnett, was found dead last month with what appeared to be a self-inflicted gunshot wound.

The Dreamliner was a pioneer in using large amounts of so-called composite materials rather than traditional metal to build the plane, including major sections like the fuselage, as the aircraft’s body is known. Often made by combining materials like carbon and glass fibers, composites are lighter than metals but, as comparatively newer materials, less is known about how they hold up to the long-term stresses of flight. Those stresses create what engineers call fatigue, which can compromise safety if it causes the material to fail.

Mr. Salehpour said he was repeatedly retaliated against for raising concerns about shortcuts he believed that Boeing was taking in joining together the pieces of the Dreamliner’s fuselage.

Debra S. Katz, a lawyer for Mr. Salehpour, said that her client raised his concerns with supervisors and tried to discuss them in safety meetings, but that company officials did not listen. Instead, she said that Mr. Salehpour was silenced and transferred to work on another wide-body aircraft, the 777. Mr. Salehpour said that after his transfer, he found additional problems with how Boeing was assembling the fuselage of the 777.

“This is the culture that Boeing has allowed to exist,” Ms. Katz said. “This is a culture that prioritizes production of planes and pushes them off the line even when there are serious concerns about the structural integrity of those planes and their production process.”

In its statement, Boeing said that it encouraged its workers “to speak up when issues arise” and that retaliation was “strictly prohibited.”

The F.A.A. interviewed Mr. Salehpour on Friday, Ms. Katz said. In response to questions about the Dreamliner, Mike Whitaker, the agency’s administrator, reiterated that the regulator was taking a hard line against the Boeing after the Alaska Airlines episode.

“This won’t be back to business as usual for Boeing,” Mr. Whitaker said in a statement. “They must commit to real and profound improvements. Making foundational change will require a sustained effort from Boeing’s leadership, and we are going to hold them accountable every step of the way.”

Mr. Salehpour said the shortcuts that he believed Boeing was taking resulted in excessive force being applied to narrow unwanted gaps in the assembly connecting pieces of the Dreamliner’s fuselage. He said that force led to deformation in the composite material, which he said could increase the effects of fatigue and lead to premature failure of the composite.

John Cox, a former airline pilot who runs a safety consulting firm, said that while composites were more tolerant of excess force than metals, it was harder to see that composites had been stressed to the point that they would fail. “They just snap,” he said.

“The catastrophic in-flight breakup, yes, that’s a theoretical possibility,” Mr. Cox said. “That’s why you’d want to have the testing done to preclude that.”

Boeing’s tests are an appropriate step, Mr. Cox said, because “if the degradation goes far enough, that could potentially lead to a catastrophic failure.”

Kitty Bennett contributed research.