On a Boeing Co. assembly line in Kansas in 2000, Prewitt saw workers drilling extra holes in the long aluminum ribs that make up the skeleton of a jetliner's fuselage. That was the only way the workers could attach the pieces because some of its pre-drilled holes didn't match those on the airframe.

Prewitt was a parts buyer, the third generation of her family to work at the sprawling Boeing factory on the outskirts of Wichita. She believed pieces going into one of the world's most advanced and popular airliners, the Boeing 737, should fit like a glove.

The assembly workers Prewitt observed were not the only ones who noted problems with parts from a key Boeing supplier, AHF Ducommun of Los Angeles. Other workers told her many pieces had to be shoved or hammered into place. And documents reviewed by The Washington Post show that quality managers reported numerous problems at Ducommun.

Whether questionable parts ended up in hundreds of Boeing 737s is the subject of a bitter dispute between the aerospace company and Prewitt and two other whistle-blowers. The two sides also have enormously different views on what that could mean for the safety of the jets.

The whistle-blower lawsuit is in U.S. District Court in Wichita. No matter how it is resolved, it has exposed gaps in the way government regulators investigated the alleged problems in aircraft manufacturing, according to documents and interviews.

Boeing said the lawsuit is without merit and there is no safety issue. Even if faulty parts landed on the assembly line, the company said, none could have slipped through Boeing's controls and gotten into the jetliners.

The three whistle-blowers contend that Boeing officials knew from their own audits about thousands of parts that did not meet specifications, allowed them to be installed and retaliated against people who raised questions. They say the parts, manufactured from 1994 to 2002, fit the Federal Aviation Administration's definition of ''unapproved'' because they lack documentation proving they are airworthy. Moreover, they say, forcing a part into place could shorten its lifespan.

Under the U.S. False Claims Act, plaintiffs who prove the government was defrauded - more than two dozen jets went to the U.S. military - could receive monetary damages along with the government.

After the whistle-blowers notified federal authorities in 2002, the FAA and the Pentagon looked into their charges. Each said its investigation cleared the airplane parts and found no reports of problems from military or civilian operators of Boeing jets. The Post's review, however, found that the FAA did not assess many of the whistle-blowers' key allegations. FAA inspectors examined only a small number of parts in the plants and did not visit any airplanes to inspect the roughly 200 types of parts questioned by the whistle-blowers.

The Pentagon and Transportation Department, in turn, relied on the FAA's work, documents show.

One reason the FAA chose not to pursue the whistle-blowers' claims, officials said, was that its engineers believed the parts in question would not present a safety risk even if they failed in flight. There has never been a crash caused by such a failure, the agency said.

But on a number of occasions, the agency has expressed concern about similar parts, albeit on the previous generation of 737s. Last year, prompted by reports from some carriers of cracks, the FAA formally alerted U.S. air carriers that fly 737s made before 1998 to inspect for possible fatigue cracks around such parts. Cracks in these areas, the FAA said, ''could result in reduced structural integrity of the frames, possible loss of a cargo door, possible rapid decompression of the fuselage.''

Boeing's corporate audit office convened a team to look into the parts problems in 2000. The 14 members included Prewitt and two others who later joined the whistle-blower lawsuit - Taylor Smith, 44, contract administrator for the new generation of 737 and other jets; and James Ailes, 53, a technical troubleshooter.

The team visited Ducommun's Gardena, Calif., plant. In its report to Boeing, the team said it found that many of the more than 500 heavy-duty manufacturing tools used by Ducommun were incorrectly calibrated, misused or not built to Boeing's specifications. Ducommun, also named in the lawsuit, declined to comment on the allegations beyond stating that the FAA and other agencies had already dismissed them.

Prewitt received a cash-and-stock bonus worth nearly $3,000 after what Boeing called her ''outstanding contribution'' to the audit. Soon, however, members of the team grew discouraged with what they saw as Boeing's reluctance to follow up on their findings. They said Boeing officials cleaned the report of details about possible airliner safety problems and violations of FAA procedures. When they raised the possibility of reporting their concerns to the FAA, they said, they were told to keep quiet or face possible legal action from Boeing.

Boeing said it did not sanitize the report, and its policies prohibit threats or retaliation against employees who raise safety questions.

In early 2002, Prewitt, Smith and Ailes sent thousands of documents supporting their case to the Justice Department. They alleged questionable parts had been installed not only on hundreds of 737s but also on some 747s, 757s, 767s and 777s and their military equivalents without the knowledge of the Air Force and Navy, the commercial airlines or the FAA. Shortly after that, in March 2002, the three workers - and one other whistle-blower who later dropped out - filed their lawsuit.

In 2003, the whistle-blowers withdrew their suit after the Justice Department declined to join. They refiled it in March 2005. By then, Ailes was still employed at what is now Spirit AeroSystems in Wichita, but Prewitt and Smith had been laid off. All three allege they received demotions and lower job evaluations because of their actions.

So at the request of the Justice Department, the FAA launched a probe in the spring of 2002. It was handled by the division that investigates parts suspected to be ''unapproved'' - ones that lack the paper trail showing they meet specifications.

In the end, the engineers narrowed their list to 11 of the ''most critical'' Ducommun commercial parts and the FAA focused its investigation on how they were being made at the time of its probe. The agency said it has no official documents explaining the decision to eliminate hundreds of parts from investigation. That did not follow procedures requiring that FAA inspectors review the manufacturing history, quantity and importance of each part that is reported as suspect and then document their findings.

The military did not examine the parts questioned by the whistle-blowers, either, apparently because officials believed the FAA was doing it. After finding nothing the first time around, last year the FAA reopened the case. The agency had received new reports about the parts from two FAA-certified experts hired by the whistle-blowers' lawyers.

The lawyers had provided four experts with the court documents and Boeing quality control reports from 1999 and 2000. All four experts, who are certified by the FAA to make decisions about aircraft engineering or airworthiness on behalf of the agency said they believed that practices at Ducommun and Boeing were seriously flawed.

Other aviation consultants said that even if FAA procedures were violated, metal parts used for reinforcement are not as critical as, say, the main landing gear.

''Sheet metal parts are necessarily pretty flexible so if they don't fit perfect as delivered, it's not a big deal to shove them into place,'' said Charles Eastlake, a professor of aerospace engineering at Embry-Riddle Aeronautical University in Daytona Beach, Fla. ''Quality-control people turn purple when they see that, but it's the way it's always been.''

Another argument holds that because planes are stripped down for major maintenance every five to seven years, any early cracks or corrosion would probably be spotted before the part could create a problem. In fact, FAA officials said their inspectors combed through records from airlines that performed such maintenance and found no reports of problems with bear straps, chords or frames. But some analysts suggest that when factory workers force together parts that are not built according to their design, it could eventually cause premature cracking.

The FAA has yet to complete its second investigation. ''We're confident we came to the right conclusions in the first case,'' said Brown, the FAA spokeswoman.