United Airlines reported Monday that it has found loose bolts and installation issues on multiple 737 MAX 9 aircraft, which were grounded following a harrowing flight in which a similar Alaska Airlines jetliner was left with a gaping hole in its side.

On Friday a door plug blew out on Alaska Airlines’ Boeing 737 Max 9 about six minutes after takeoff from Portland, Ore., on a plane that had been bound for Ontario, Calif. The incident, which occurred as Flight 1282 was climbing at about 16,000 feet, caused a rapid loss of cabin pressure that pulled the clothes off a teenage boy and caused oxygen masks to drop from the ceiling. Pilots made a safe emergency landing. Alaska Airlines officials said several passengers suffered injuries that required medical attention, but all have since been medically cleared.

The Alaska Airlines incident did not involve a Hawaii flight, but the nearly brand-new plane involved in the fuselage blowout has flown between the islands and the mainland before. Alaska said it took delivery of the plane Oct. 31.

The incident prompted a National Transportation Safety Board investigation and has caused the Federal Aviation Administration to issue a temporary grounding of some 171 of the Boeing 737 MAX 9 aircraft operated by U.S. airlines or flown in the United States by foreign carriers.

As a result, Alaska and United, which fly the 737 MAX 9 planes, have delayed or canceled hundreds of flights. No other U.S. airlines operate that model of the Boeing 737.

Alaska said in a statement Monday, “We recognize that additional questions remain about the details surrounding Flight 1282 that we are unable to address at this time. Because this is an active investigation, we must receive permission from the NTSB to provide information about the aircraft and its prior maintenance. We will provide information as soon as the NTSB permits us to do so.”

United’s preliminary findings are in addition to a report from Jennifer Homendy, chair of the National Transportation Safety Board, that warning lights were triggered on three prior Alaska flights on that plane, including each of the two days before the incident.

Homendy said maintenance crews checked the plane and cleared it to fly, but the airline decided not to use it for the long route to Hawaii over water so that it “could return very quickly to an airport” if the warning light reappeared.

Alaska’s decision to keep flying the aircraft on land is raising questions about whether the jet should have been flying at all. But Homendy cautioned that the pressurization light might be unrelated to Friday’s incident in which a plug covering an unused exit door blew off the Boeing 737 Max 9 as it cruised about 3 miles over Oregon.

At a news conference Sunday night, she provided new details about the chaotic scene that unfolded on the plane. The explosive rush of air damaged several rows of seats and pulled insulation from the walls. The cockpit door flew open and banged into a lavatory door.

The force ripped the headset off the co-pilot, and the captain lost part of her headset. A quick reference checklist kept within easy reach of the pilots flew out of the open cockpit, Homendy said.

The NTSB said the cockpit voice recorder for Flight 1282 had been erased as it overwrites every two hours. Homendy called for the FAA to require planes to retain cockpit voice recorder information for 25 hours.

Two cellphones that appeared to have belonged to passengers on Friday’s terrifying flight were found on the ground. One was discovered in a yard, the other on the side of a road. Both were turned over to the NTSB.

The NTSB has said that the lost door plug was found Sunday near Portland in the backyard of a home. Investigators will examine the plug, which is 26 by 48 inches and weighs 63 pounds, for signs of how it broke free.

In the immediate aftermath of the incident, Alaska Airlines CEO Ben Minicucci took precautionary actions in advance of the FAA directive. Following the incident Friday, he announced that the airline was grounding its fleet of 737 MAX 9s and would not return them to service before performing full maintenance and safety checks.

On Saturday, Alaska said that it had inspected 18 of the 65 aircraft in its 737 MAX 9 fleet and that they had been cleared to fly. However, before the end of Saturday, Alaska said those planes had been pulled from service “until details about possible additional maintenance work are confirmed with the FAA.”

The FAA said Monday that it has approved a method to comply with the FAA’s Boeing 737-9 emergency airworthiness directive, and it has been provided to the affected operators. A focus of the inspections is likely to be on the plugs used to seal an area set aside for extra emergency doors that are not required on United and Alaska MAX 9s.

The FAA said in a statement, “The FAA’s priority is always keeping Americans safe. In that spirit, Boeing 737-9 aircraft will remain grounded until operators complete enhanced inspections which include both left and right cabin door exit plugs, door components, and fasteners. Operators must also complete corrective action requirements based on findings from the inspections prior to bringing any aircraft back into service.”

Alaska said its technicians have prepped each aircraft “to be immediately ready” for the required inspections. The airline said inspections can begin once the FAA approves operators’ inspection processes to ensure compliance (called an Alternate Methods of Compliance, or AMOC), and Alaska develops a detailed inspection instructions and processes for its maintenance technicians to follow.

“As these steps remain pending, we continue to experience disruption to our operation with these aircraft out of service. As of 8:30 a.m. Pacific, we have cancelled roughly 140 flights for Monday due to the 737-9 MAX grounding,” Alaska said.

United, which has 79 of the 737 MAX 9 aircraft, said in an email Monday that it was still awaiting final FAA approval on the full inspection process.

“We’re working to return our Boeing 737 MAX 9s to service in the days ahead,” United said.

As of Monday, United said it had canceled 200 flights on 737 MAX 9 aircraft for which service remains suspended.

“We expect significant cancellations on Tuesday as well,” United said. “We have been able to operate some planned flights by switching to other aircraft types, avoiding about 30 cancellations each on Monday and Tuesday.”

Keith Vieira, principal of KV & Associates, Hospitality Consulting, said he is unaware of major visitor industry disruptions as a result of the incident and grounding.

According to the airline tracking site FlightAware, Alaska on Monday had canceled eight flights to Honolulu and three to Maui. It had six delayed flights in Honolulu and one on Maui.

United canceled two Honolulu flights and one Maui flight. Five of its Honolulu flights were delayed, along with two Maui flights.

Vieira said he has not heard reports of major Hawaii hotel cancellations, or stranded passengers, either.

“If their flight was canceled, I think most people would try to come to Hawaii on another flight,” he said. “If people needed to stay over, we could accommodate them. First quarter is soft.”

Vieira said he expects the incident will be “short-lived,” although he said Hawaii is more vulnerable to airline concerns as the majority of Hawaii’s visitors arrive on planes.

“I think there will be people who are concerned, but I don’t think we’ll see major tourism changes because of it,” he said.

Vieira opined that the incident is unlikely to jeopardize Alaska’s acquisition of Hawaiian Airlines.

Seattle-based Alaska Airlines is buying kamaaina company Hawaiian Airlines in a $1.9 billion deal that was announced in December.

Subject to regulatory and shareholder approvals, the deal is expected to close over a 12- to 18-month period. If approved, the deal would mean that Alaska, the nation’s fifth-largest airline, would grow to a fleet of 365 airplanes serving 138 destinations.

“I think it will have moved on by that point in the acquisition process,” he said.

Alaska has not released results of the preliminary inspections that it has performed since Friday’s incident.

However, United said in a statement Monday, “Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug — for example, bolts that needed additional tightening.”

United said its technical operations teams will remedy these findings to safely return the aircraft to service.

“Each inspection will include a team of five United technicians working for several hours on each aircraft,” United said. “We’ll learn more about the exact amount of time each inspection requires as we do more of them.”

United said it began preparing for inspections Saturday by removing the inner panel to access the door plug and visually inspecting the area while awaiting final instructions.

To access the door plug, United said it must remove two rows of seats and the sidewall liner.

“This has already been done on most Max 9s,” United said.

The airline said it planned to inspect and verify proper installation of the door and frame hardware.

“We’ll open the door and inspect the area around the door and seal,” United said. “We’ll then resecure the door, ensuring proper fit and security.”

United said it would document and correct discrepancies found during inspections before returning each 737 MAX 9 plane into service.

Boeing CEO Dave Calhoun called a companywide webcast for today to talk about the incident with employees and senior leadership.

“When serious accidents like this occur, it is critical for us to work transparently with our customers and regulators to understand and address the causes of the event, and to ensure they don’t happen again,” Calhoun wrote in a message to employees Sunday. “This is and must be the focus of our team right now.”

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The Associated Press and Star-Advertiser staff writer Peter Boylan contributed to this story.