At the factory in Wichita, Kan., where Spirit AeroSystems builds the fuselage for the Boeing 737 Max, management would sometimes throw a pizza party to celebrate a drop in the number of problems reported on the line.

But Joshua Dean says many of the workers knew something was off.

"We're having a pizza party because we're lowering defects," said Dean, a former quality auditor at the factory. "But we're not lowering defects. We just ain't reporting them, you know what I mean?"

Now federal investigators are looking more closely at Spirit AeroSystems to understand what went wrong with the door panel that blew off an Alaska Airlines Boeing 737 Max 9 in midair last month — the latest chapter in the long and troubled relationship between the two companies.

Dean says his father and grandfather had worked at the same Wichita factory, and he took his job as an auditor seriously. So Dean got frustrated with what he describes as a "culture" that pressured employees not to report defects in order to get planes out of the factory faster.

"Now, I'm not saying they don't want you to go out there and inspect a job. You know, they do," Dean said. "But if you make too much trouble, you will get the Josh treatment. You will get what happened to me."

Dean was fired last April — in retaliation, he says, for flagging improperly drilled holes in fuselages.

"I think they were sending out a message to anybody else," Dean said. "If you are too loud, we will silence you."

Dean has given statements as part of a shareholder lawsuit against Spirit AeroSystems that alleges an "excessive" numbers of defects at the Kansas factory, although he's not a plaintiff.

A Spirit spokesman says the company strongly disagrees with those allegations, and is fighting the case in court.

Now the FAA is sending a team of roughly two dozen aviation safety inspectors to conduct reviews at Spirit's factory in Wichita, and at Boeing's 737 Max factory in Renton, Wash.

And late Sunday, Boeing announced that about 50 jets will need additional work before they can leave the factory because of problems with improperly drilled holes. Boeing says the production errors are "not an immediate flight safety issue."

"A member of our team identified an issue that does not conform to engineering standards," said Spirit AeroSystems spokesman Joe Buccino in a statement. "We are in close communication with Boeing on this matter," he said.

The two companies have clashed before over costs and quality. Last year, Spirit reported two other embarrassing and expensive production problems that were not related to the door plug.

Spirit fired its CEO last October, and brought in Pat Shanahan, a former Boeing executive, to run the company.

"The mindset I have is that we can eliminate all defects," Shanahan said on a call with analysts and investors last November. "I think you'll see us improve quite a bit in this area.

In a deal reached last year, Boeing agreed to give Spirit $100 million to build its production capacity, while Spirit pledged to improve its quality control.

"I believe that we'll be able to stabilize here and meet Boeing's demands in 2024," Shanahan said on the earnings call.

But Spirit and Boeing suffered a major setback just a few days into the new year, when a door plug blew off a 737 Max 9 jet in midair.

The chair of the National Transportation Safety Board, Jennifer Homendy, told NPR's Morning Edition investigators are focused on four bolts that are supposed to hold the door plug in place.

"We don't know whether those bolts themselves also fractured, were loose or whether they weren't even installed on the door," Homendy said.

Investigators have not yet said what they think happened to those bolts. But an anonymous whistleblower who claims to be a Boeing insider offered an explanation.

The whistleblower says the door plug had to be removed for maintenance because the fuselage arrived from Spirit's Kansas factory with damaged and improperly installed rivets.

That's not unusual, according to the whistleblower. They say Boeing has discovered "a hideously high and very alarming number" of defects after the fuselages were delivered to its factory in Washington.

The whistleblower also describes major communication problems between mechanics from Spirit and Boeing. The whistleblower, who appears to have access to Boeing's internal records, wrote that the door plugs "were not installed when Boeing delivered the plane, our own records reflect this."

Quality control problems at the two companies come as no surprise to longtime industry analyst Richard Aboulafia, because Boeing has been aggressively pushing its suppliers to cut costs for years.

"You had the people at the top focused on numbers, money," said Aboulafia, the managing director at AeroDynamic Advisory. "Basically, everyone was subject to rigorous cost cuts without an awareness of what this would do to technical execution."

The company that's now Spirit AeroSystems used to be part of Boeing, until it was sold off in 2005. Boeing leaders now concede that they may have outsourced too many parts of the manufacturing chain.

"Did it go too far? Yeah, probably did. Now it's here and now, and now I've got to deal with it," Boeing CEO Dave Calhoun said in an interview with CNBC last week. "And yes, the subject of how we interact with all of our suppliers at large, that will be a subject that we will be working out for quite a long time."

An example of that came on Sunday, when Boeing announced the latest production problems with the fuselages of some 737 jets.

That wasn't all Stan Deal, the head of Boeing's commercial airplanes division, announced.

"We recently instructed a major supplier to hold shipments until all jobs have been completed to specification," Deal wrote in a letter to Boeing employees. He said the delay in shipment would affect the company's production schedule, but would also help improve overall quality.

"This is what we mean when we say that we will go slow to get it right," Deal said.