NTSB: Alaska Airlines was missing four bolts that secured door plug at the time of blowout

The National Transportation Safety Board believes bolts securing a door plug were missing on Alaska Airlines flight that suffered midair blowout.

A panel called a door plug blew off an Alaska Max 9 as it flew 3 miles (5 kilometers) above Oregon on Jan. 5. The blowout left a hole in the side of the plane, but pilots were able to return to Portland and land safely.

The report included a photo from Boeing, which worked on the panel, which is called a door plug. In the photo, three of the four bolts that prevent the panel from moving upward are missing. The location of the fourth bolt is obscured.

The investigators said that the lack of certain damage around the panel indicates that all four bolts were missing before the plane took off from Portland, Oregon.

Without the bolts, nothing prevented the panel from sliding upward and detaching from "stop pads" that secured it to the airframe.

The preliminary report said the door plug, installed by supplier Spirit AeroSystems, arrived at Boeing’s factory near Seattle with five damaged rivets around the plug. A Boeing crew replaced the damaged rivets, which required them to remove the four bolts to open the plug.

The NTSB did not declare a probable cause for the accident – that will come at the end of an investigation that could last a year or longer.

Alaska Air Flight 1282 was traveling to Ontario, California, but had to make an emergency landing in Portland, Oregon. According to flight data, before the incident took place, the plane climbed to 16,000 feet (4,876 meters). Jennifer Homendy, the chair of the National Transportation Safety Board (NTSB), explained that the door fell off over the Portland suburb of Cedar Hills.

Boeing has released a statement in response to the NTSB's report along with actions the company is taking. 

"Whatever final conclusions are reached, Boeing is accountable for what happened," Boeing President and CEO Dave Calhoun said in a statement. "An event like this must not happen on an airplane that leaves our factory. We simply must do better for our customers and their passengers."

Spirit AeroSystems, which Boeing spun off as a separate company nearly 20 years ago, said in a statement that it was reviewing the NTSB preliminary report and was working with Boeing and regulators "on continuous improvement in our processes and meeting the highest standards of safety, quality and reliability."

Safety experts have said the accident could have been catastrophic if the Alaska jet had reached cruising altitude. The decompression in the cabin after the blowout would have been far stronger, and passengers and flight attendants might have been walking around instead of being belted into their seats.

Passengers were met with horror.

One traveler, who was on board the flight with her two grandchildren, shared her experience on "America Reports," Monday.

"I knew there was a sense of a boom. I don't know if I felt that or heard it," Vicki Kreps said.

"There was like a mist in the air immediately following. My brain was processing it as smoke, and so I kind of lowered my mask slightly to smell and to look around. And that's when, over my left shoulder, I could see that there was a hole in the plane."

"I sort of hear a swishing above me, my brain processed as above me, which I think may have been depressurization. Then my body is sort of thrown forward, back again with a gust of wind. The oxygen masks fall, and, the pilot is on saying, 'this is an emergency. We've experienced decompression.' And he told us, ‘get your own mask on first before helping others,’" Kreps said.

Kreps and her two young grandchildren were sitting in row 19, and the door plug blew out near row 26.

"So once kids were masked and settled in, and I took their hands, we did say a prayer, that if we were going to meet Jesus today, that we knew we were going to be safely in his arms," Kreps added.

The incident is the latest involving the Boeing 737 MAX, which plays a critical role for the company but has been hit by a number of setbacks since it was introduced as the successor to the 737 Next Generation six and a half years ago.

Alaska Airlines and United Airlines, the two U.S. air carriers that use the 737 MAX 9, grounded their fleets to inspect their aircraft while the Federal Aviation Administration (FAA) and National Transportation Safety Board carried out an investigation into the incident which resulted in hundreds of flights getting canceled. The MAX 9 involved in the incident had been restricted from long flights over water, such as to Hawaii, after Alaska reported pressurization alerts on prior flights.

"I would have rather not have experienced what I experienced," Kreps told Fox News. "I was so grateful to the crew and to everyone that cared for us while we were on the plane and experiencing, getting us back on the ground as quickly as they possibly could. That time passed very quickly, and we were back on the ground."

The Associated Press and FOX News contributed. This story was reported from Los Angeles.