WASHINGTON — The Federal Aviation Administration recommends that airlines inspect the door plugs on certain Boeing 737s that are older than the Max 9 jetliner that suffered a blowout of a similar panel during a flight this month.

The FAA said door plugs on one older version of the 737, called the 737-900ER, are identical in design to those on the Max 9, and some airlines “have noted findings with bolts during the maintenance inspections.”

The FAA issued a safety alert late Sunday. As soon as possible, airlines should visually inspect four places where a bolt, nut and pin secure the door plug to the plane, the agency said.

The door plugs are panels that seal holes left for extra doors when the number of seats is not enough to trigger a requirement for more emergency-evacuation exits. From inside the plane, they look like a regular window.

The FAA said that 737-900ERs have logged 3.9 million flights without any known issues involving the door plugs. By contrast, the Max 9 that suffered the blowout had made only 145 flights.

One of the two door plugs on an Alaska Airlines Boeing 737 Max 9 flew off the plane in midflight on Jan. 5. Passengers and investigators have described a violent decompression as air rushed out of the pressured cabin at 16,000 feet over Oregon.

The plane made an emergency landing with a hole in its side, and no serious injuries were reported. On a mostly full plane, no one was sitting in the two seats nearest the panel that blew out.

The FAA has grounded all Max 9s with door plugs instead of regular doors in the back of the cabin. Alaska and United Airlines are the only U.S. carriers to use the plane.

Besides grounding 171 Max 9 jets, the FAA is investigating quality-control measures at Boeing and its suppliers and has increased its oversight of the aircraft maker. The National Transportation Safety Board is investigating the Oregon incident.

“We fully support the FAA and our customers in this action,” a Boeing spokeswoman said in a statement Monday.

Boeing delivered about 500 737-900ERs between 2007 and 2019 – mostly to three U.S. airlines -- and about 380 have door plugs instead of exits.

Alaska said Monday that it began inspecting its 737-900ERs “several days ago” and has turned up no issues. The Seattle-based airline, which has canceled hundreds of flights because of the Max 9 grounding, said it expects no disruption from the inspections of its 737-900ERs.

United said it began inspecting the earlier Boeing model last week and expects to finish the reviews this week with no disruptions for passengers. Delta Air Lines, with 130 planes affected by the new FAA recommendation, said it was inspecting all 130 of its 737-900ERs and did not expect any impact on operations.