An advisory light on the Alaska Airlines plane that lost a piece of its fuselage last week had come on during previous flights, preventing the aircraft from being used on long flights over water, the National Transportation Safety Board said.

Additionally, the flight crew and attendants described the atmosphere aboard last Friday's Alaska Airlines-operated Boeing 737 Max 9 flight as chaotic, "loud" and "very violent" once a door plug flew off and left a vast hole in the side of the plane. The flight carrying 171 passengers and six crew safely returned back to Portland, Ore. There were no injuries.

A door plug is used to fill a doorway and held together by 12 stop fittings, which prevent the door plug from becoming dislodged. In this case, the plug was not used for a functional door, NTSB Chairperson Jennifer Homendy said at a Sunday news conference.

The door plug was found in the backyard of a Portland schoolteacher named Bob, Homendy said. Two cellphones were also found — one on the side of the road and the other in a yard, she added.

An advisory light came on during 3 previous flights

On three flights prior to Friday's, the plane's auto pressurization fail light came on, Homendy said.

The flights happened on Dec. 7 of last year, and then on Jan. 3 and Jan. 4 — just prior to last Friday's flight. The light coming on is "very benign," Homendy said, and it was tested by maintenance crews and reset.

"We don't know that there was any correlation of the two," Homendy said. "It could be entirely separate."

Alaska Airlines then temporarily restricted the plane from being flown over water to Hawaii, so that it could be easily accessible to an airport. The airline requested that maintenance crews examine the light. However, the requests had not been fulfilled before the plug came off.

The moment the door plug broke

When the plug flew off, the flight crew reported hearing a loud bang, Homendy said at the news conference.

The crew immediately put on their oxygen masks. Their quick reference checklist flew out of the cockpit door, which had flung open and jammed the door to the bathroom, which was empty. Instead, the captain handed the quick reference handbook to the first officer, who had jolted forward, causing her headset to come off. The crew put their oxygen masks on and turned on the speakers to alert those in the cabin.

The circuit breaker to the cockpit voice recorder was not pulled, so the recorder was therefore empty, Homendy said.

"Communication was a serious issue," Homendy said. "The flight attendants reported that it was difficult to get information from the flight deck, and the flight deck was having trouble also communicating."

The NTSB has interviewed two flight attendants who were at the front of the plane and will interview the two who were in the back of the plane on Monday.

The plane seats 178 people. The 171 people on board included four unaccompanied minors and three babies, who were in the laps of caregivers, Homendy said.

Homendy applauded the work of the flight crew and attendants.

"After this explosive event occurred suddenly, the flight attendants were very focused on what was going on with those children," Homendy said. "Were they safe? Were they secure? Did they have their lap belts on, and did they have their masks on? And they did."

Homendy pointed out that the NTSB, as well as the Federal Aviation Administration and Alaska Airlines, encourages caregivers to purchase seats for infants under age 2 and safely strap them into a car seat.

Damage to the interior

The plug was positioned in Row 26, on the left side of the plane. No one was seated in the two seats adjacent to the door, seats 26A and 26B. The headrests on both of those seats are gone and the tray table on the back of 26A is missing. The seats have been sent to NTSB to be inspected.

There was a lot of damage to the paneling and trim around the area. The seal to the window was undamaged, Homendy said.

"Those are all not critical to the structure of the aircraft, so I just want to emphasize that," she said.

The NTSB is investigating the functionality of an oxygen mask that was still in its ceiling panel. Homendy said it either did not deploy or someone must have put it back up after the incident.

There was additional damage in rows 1 through 4, 11 and 12, 25 through 27 and 31 through 33. There was no damage to the exterior of the plane, Homendy said.

What's next in the investigation

NTSB teams spent Sunday documenting damage to the frame. They have looked for paint transfers and are sending some components, such as the stop fittings, to a laboratory to be examined, where the NTSB will search for things like fractures and shears under a microscope.

The corresponding door plug on the right side of the plane will also be inspected.

The FAA has grounded 171 models of the Boeing 737 Max 9 planes and mandated they be inspected immediately. Alaska Airlines has grounded all of its models of that plane.

"Safety is our top priority and we deeply regret the impact this event has had on our customers and their passengers," Boeing said Saturday. "We agree with and fully support the FAA's decision to require immediate inspections of 737-9 airplanes with the same configuration as the affected airplane. In addition, a Boeing technical team is supporting the NTSB's investigation into the Jan. 5 accident. We will remain in close contact with our regulator and customers."

"My heart goes out to those who were on this flight — I am so sorry for what you experienced," Alaska Airlines CEO Ben Minicucci said in a statement. "We are working with Boeing and regulators to understand what occurred tonight, and will share updates as more information is available."