Four bolts used to secure the panel that ultimately blew up an Alaska Airlines plane during a flight last month were removed (and appear not to have been replaced) at Boeing’s factory in Renton, Washington, according to a preliminary report released Tuesday by the National Transportation Safety Board.

The panel, known as a door plug, was opened to repair damaged rivets in the plane’s fuselage, according to Boeing records. The report did not say who removed the bolts holding the door stopper in place. But the safety board said it appeared that not all of the bolts were replaced once the door was reinstalled on the plane after the rivets were repaired.

As evidence, the NTSB provided a photograph of the door plug after it was reinstalled but before the interior of the plane was restored. In the picture, it looks like three of the four screws are missing. The fourth bolt location is covered with insulation.

The report said the image was attached to “a text message between Boeing team members on September 19, 2023.” Boeing employees “were discussing interior restoration after rivet overhaul was completed during second shift operations that day,” according to the report.

The safety board said there was no evidence that the plug had been opened again after leaving the Boeing factory. The plane was delivered to Alaska Airlines at the end of October.

The report intensifies scrutiny on Boeing, which has been struggling for weeks to contain fallout from the incident, and raises new questions about whether the company did enough to improve safety after two fatal crashes of 737 Max 8 planes in 2018 and 2019. It also answers critical questions about why the door plug came loose shortly after Alaska Airlines Flight 1282 took off from Portland International Airport in Oregon.

In a statement, Dave Calhoun, Boeing’s chief executive, said: “Whatever the final conclusion is reached, Boeing is responsible for what happened.”

“An event like this should not occur on a plane that leaves our factory,” he added. “We simply must do better for our customers and their passengers. “We are implementing a comprehensive plan to strengthen the quality and trust of our stakeholders.”

The NTSB eliminated other possible reasons for the faulty door stopper installation. The part was manufactured in Malaysia in March and received by Spirit AeroSystems, a Boeing supplier in Wichita, Kansas, that makes Max airframes, in May, according to the report. While the safety board said Spirit had detected a minor problem with the door plug’s “flush seal,” the report found that the problem had not required any additional manufacturing work and that Spirit had not indicated other notifications of quality for the plug.

Spirit spokesman Joe Buccino said: “We remain focused on working closely with Boeing and our regulators to continually improve our processes and meet the highest standards of safety, quality and reliability.”

The fuselage was then shipped to Boeing on August 20 and arrived at the Renton factory on August 31, according to the report. There, damaged rivets, which are often used to join and secure parts on airplanes, were pointed out on September 1. Once the plug was removed to access the rivets, employees at Spirit AeroSystems in Renton completed the repairs.

After the plane was delivered to Alaska Airlines, wireless Internet equipment was also installed in Oklahoma City from November 27 to December 7. But the contractor who did that work, AAR, said he had “modified approximately 60” Alaska Airlines 737 Max. 9 aircraft and had not had to remove any door plugs to perform that work, according to the report.

The safety board said its investigation would continue to look at what documents had been used “to authorize the opening and closing” of the door stopper.

Almost immediately, the Alaska Airlines incident led the Federal Aviation Administration to ground some Max 9 planes, snarling flight schedules for days at Alaska and United Airlines, the two U.S. airlines that fly the model.

“This incident should never have happened and cannot happen again,” the FAA said in a statement Tuesday after the safety board report was released.

The FAA has also indefinitely limited Boeing’s ambitious plans to increase production of all Max planes, plunging the company into uncertainty. The company had planned to produce 42 planes per month this year and 50 per month next year, but will instead remain stable at 38, possibly for many months. Boeing executives last week declined to provide a financial forecast for the year, citing the incident and the need to focus on safety.

Furious airline executives have taken the rare step of publicly criticizing Boeing and expressing doubt that it can deliver the planes they ordered on time.

The incident and its knock-on effects have plunged Boeing, one of the world’s two largest airplane makers, into a familiar position: trying to navigate through a crisis with unknown financial and reputational costs. Just five years ago, after two Max 8 crashes killed nearly 350 people, the company spent billions of dollars to make its planes safer and repair its reputation. Those accidents were caused by a failure in the plane’s flight stabilizer system.

With Boeing again on its heels, it is quick to assure its customers, regulators and members of Congress that it is squarely focused on improving quality control. Mr. Calhoun visited Spirit in Wichita. Boeing also held an event where employees at the Renton factory stopped work for a day to attend quality sessions. And he has promised to reward employees “for speaking up to slow things down if that’s what’s necessary.”

But even as it tries to resolve its problems, Boeing said Sunday that last week a supplier had encountered a new problem with the fuselages of dozens of unfinished 737 Max planes. The supplier discovered that “two holes may not have been drilled exactly to our requirements.”

Although it did not name the supplier, a Spirit spokesperson said a member of its team had identified an issue last week that did not meet engineering standards. Boeing said the problem would force about 50 planes to be reworked, delaying their delivery.

In a call with analysts on Tuesday, Spirit AeroSystems CEO Patrick Shanahan said it was increasing the number of inspections it performed, along with those conducted by Boeing.

Also on Tuesday, Mike Whitaker, the FAA’s top official, told a House panel that the agency would step up its ground presence monitoring Boeing’s aircraft production.

“Going forward, we will have more troops on the ground closely examining and monitoring production and manufacturing activities,” Mr. Whitaker told the Transportation and Infrastructure Committee’s Aviation Subcommittee.

In addition to limiting Boeing’s production ramp-up, the agency has opened an investigation into the planemaker’s compliance with safety standards. He also began an audit to examine the company’s production of the Max, which Whitaker said would take six weeks.

He said the agency had deployed about two dozen inspectors to Boeing and another half dozen to Spirit.

Santul Nerkar contributed with reports.