Four bolts used to secure the panel that ultimately blew off an Alaska Airlines plane during a flight last month were removed — and appear not to have been replaced — at Boeing’s factory in Renton, Wash., 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 on the plane’s fuselage, according to Boeing’s records. The report did not say who removed the bolts keeping the door plug in place. But the safety board said it appeared that not all the bolts were put back once the door was reinstalled on the plane after the rivets had been repaired.

As evidence, the N.T.S.B. provided a photograph of the door plug after it was reinstalled but before the plane’s interior was restored. In the image, three of the four bolts appear to be missing. The location of the fourth bolt is covered with insulation.

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

The safety board said there was no evidence that the plug was opened again after it left Boeing’s factory. The plane was delivered to Alaska Airlines at the end of October.

The report intensifies the scrutiny on Boeing, which has been scrambling for weeks to contain the fallout from the incident, and it raises fresh 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 detached 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 final conclusions are reached, Boeing is accountable for what happened.”

“An event like this must not happen on an airplane 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 quality and the confidence of our stakeholders.”

The N.T.S.B. eliminated other possible reasons for the door plug’s faulty installation. The piece was manufactured in Malaysia in March and received by Spirit AeroSystems, a Boeing supplier in Wichita, Kan., that makes Max fuselages, in May, the report said. While the safety board said Spirit had detected a minor issue with the door plug’s “seal flushness,” the report found that the issue had not required any further manufacturing work and that Spirit had indicated no other quality notifications for the plug.

Joe Buccino, a spokesman for Spirit, said, “We remain focused on working closely with Boeing and our regulators on continuous improvement in our processes and meeting the highest standards of safety, quality and reliability.”

The fuselage was then shipped to Boeing on Aug. 20, arriving at the Renton factory on Aug. 31, the report said. There, the damaged rivets — which are often used to join and secure parts on planes — were flagged on Sept. 1. Once the plug was removed for access to the rivets, Spirit AeroSystems employees in Renton completed the repairs.

After the plane was delivered to Alaska Airlines, it also had wireless internet equipment installed in Oklahoma City from Nov. 27 to Dec. 7. But the contractor that did that work, AAR, said it had “modified approximately 60” Alaska Airlines 737 Max 9 planes and had not had to remove any door plugs to do 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 plug.

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

“This incident should have never happened, and it cannot happen again,” the F.A.A. said in a statement on Tuesday after the safety board’s report was released.

The F.A.A. has also indefinitely limited Boeing’s ambitious plans to increase production of all Max jets, miring the company in uncertainty. The company had planned to churn out 42 jets a month this year and 50 a month next year, but it will instead hold steady at 38, possibly for many months. Boeing executives declined last week to provide a financial forecast for the year, citing the incident and a need to focus on safety.

Furious airline executives have taken the rare step of criticizing Boeing publicly and expressing doubt that it will be able to deliver on time the airplanes they ordered.

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

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

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

Though he did not name the supplier, a spokesman for Spirit said a member of its team had identified an issue within the past week that did not conform to engineering standards. Boeing said the problem would force Boeing to rework about 50 planes, delaying their delivery.

On a call with analysts on Tuesday, the chief executive of Spirit AeroSystems, Patrick Shanahan, said it was increasing the number of inspections it conducted, along with the ones done by Boeing.

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

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

In addition to limiting Boeing’s production increase, the agency has opened an investigation into the plane maker’s compliance with safety standards. It also began an audit looking at the company’s production of the Max, which Mr. Whitaker said would take six weeks.

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

Santul Nerkar contributed reporting.