United Airlines said Monday it found loose bolts and other “installation issues” on a part of some Boeing 737 Max 9 jets that were inspected after a mid-flight blowout on a similar Alaska Airlines jet Friday.

The inspections are focused on plugs used to seal an area set aside for extra emergency doors that are not required on United and Alaska Max 9s. That plug is the part that blew off the Alaska plane as it cruised 16,000 feet over Oregon.

“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,” Chicago-based United said.

The Federal Aviation Administration grounded all Max 9s operated by Alaska and United and some flown by foreign airlines after a terrifying flight on Friday night.

The Boeing jetliner that suffered an inflight blowout over Oregon was not being used for flights to Hawaii after a warning light that could have indicated a pressurization problem lit up on three different flights.

Alaska Airlines decided to restrict the aircraft from long flights over water so the plane “could return very quickly to an airport” if the warning light reappeared, Jennifer Homendy, chair of the National Transportation Safety Board, said Sunday.

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 three miles over Oregon.

On Monday, the FAA approved guidelines for inspecting the door plugs on other Max 9 jets and repairing them, if necessary. That move could speed the return to service of the 171 planes that the FAA grounded.

Alaska has 64 other Max 9s, and United Airlines owns 79 of them. No other U.S. airlines operate that model of the Boeing 737.

Shares of The Boeing Co. fell 8% and those of Spirit AeroSystems, which builds the fuselage for Boeing’s 737 Max, tumbled 11% Monday, the first day of trading since the incident occurred. Shares of Alaska Airlines were nearly unchanged after slumping earlier in the session.

Passengers said it was a frightening ride.

“All the oxygen masks deployed instantly and everyone got those on,” Evan Smith, an attorney traveling on the plane, told KATU-TV.

Evan Granger, who was sitting in front of the blowout, told NBC News that his “focus in that moment was just breathe into the oxygen mask and trust that the flight crew will do everything they can to keep us safe.”

“I didn’t want to look back and see what was happening.”

In this photo released by the National Transportation Safety Board, NTSB Investigator-in-Charge John Lovell examines the fuselage plug area of Alaska Airlines Flight 1282 on Sunday, Jan. 7, 2024, in Portland, Ore. A panel used to plug an area reserved for an exit door on the Boeing 737 Max 9 jetliner blew out Friday night shortly after the flight took off from Portland, forcing the plane to return to Portland International Airport. (National Transportation Safety Board via AP)
This photo released by the National Transportation Safety Board shows a gaping hole where the paneled-over door had been at the fuselage plug area of Alaska Airlines Flight 1282 on Sunday, Jan. 7, 2024, in Portland, Ore. A panel used to plug an area reserved for an exit door on the Boeing 737 Max 9 jetliner blew out Jan. 5, shortly after the flight took off from Portland, forcing the plane to return to Portland International Airport. (National Transportation Safety Board via AP)