PORTLAND, Ore. — Alaska Airlines again grounded all of its Boeing 737 Max 9 jetliners on Sunday after federal officials indicated further maintenance might be required to assure that another inflight blowout like the one that damaged one of its planes doesn’t happen again.

The airline had returned 18 of its 65 737 Max 9 aircraft to service on Saturday following inspections that came less than 24 hours after a portion of one plane’s fuselage blew out three miles above above Oregon on Friday night. The depressurized plane, which was carrying 171 passengers and six crew members, returned safely to Portland International Airport with no serious injuries.

The airline said in a statement that the decision was made after receiving a notice from the Federal Aviation Administration that additional work might be needed. Other versions of the 737 are not affected.

“These aircraft have now also been pulled from service until details about possible additional maintenance work are confirmed with the FAA. We are in touch with the FAA to determine what, if any, further work is required before these aircraft are returned to service,” the airline said.

The FAA had ordered the grounding of some 737 Max 9s on Saturday until they could be inspected, a process that takes about four hours. The world’s airlines are currently operating about 171 737 Max 9s globally.

The aircraft make up about 20% of the Alaska Airlines’ fleet. As of midday, Alaska had canceled about a fifth of its Sunday flights, according to FlightAware.com. United Airlines, which also grounded its Max 9s, had about a 10% cancellation rate on Sunday.

The National Transportation Safety Board is investigating Friday’s accident and is still looking for the door from the paneled-over exit that blew out. Some planes, due to smaller capacity, panel over what would have been an exit door.

The NTSB has a good idea of where it landed, near Oregon Route 217 and Barnes Road in the Cedar Hills area west of Portland, NTSB Chair Jennifer Homendy said at a news conference late Saturday.

“If you find that, please, please contact local law enforcement,” she said.

It was extremely lucky that the airplane had not yet reached cruising altitude, when passengers and flight attendants might be walking around the cabin, Homendy said.

“No one was seated in 26A and B where that door plug is, the aircraft was around 16,000 feet and only 10 minutes out from the airport when the door blew,” she said. The investigation is expected to take months.