The Boeing Co. 737 MAX airplane stands on the production line at the company's manufacturing facility in Renton, Washington, U.S., on Monday, Dec. 7, 2015. Boeing Co.'s latest 737 airliner is gliding through development with little notice, and that may be the plane's strongest selling point. The single-aisle 737 family is the company's largest source of profit, and the planemaker stumbled twice earlier this decade with tardy debuts for its wide-body 787 Dreamliner and 747-8 jumbo jet. Photographer: David Ryder/Bloomberg via Getty Images

(CNN) — The US Federal Aviation Administration has urged airlines to inspect so-called door plugs on an earlier version of Boeing 737 airplanes, after one blew out of the side of the newer Max model during an Alaska Airlines flight earlier this month.

The FAA on Sunday said airlines operating the Boeing 737-900ER model should “visually inspect mid-exit door plugs to ensure the door is properly secured,” referring to the panel that plugs the hole where a mid-plane exit would be.

“The Boeing 737-900ER is not part of the newer Max fleet but has the same door plug design,” the US aviation regulator added in what is known as a safety alert for operators.

Boeing’s shares fell almost 3% in premarket trade Monday. The safety alert on a plane that has been in service for almost two decades will deal a fresh blow to the company, which has faced repeated quality and safety issues with its aircraft in recent years, leading to the long-term grounding of some jets and delayed deliveries of others.

The planemaker’s stock has declined about 14% since a door plug on one of its Max 9 airplanes shot out from the side of the fuselage only a few minutes into a flight from Portland, Oregon, to Ontario, California, forcing the pilot to make an emergency landing.

In a statement shared with CNN, Boeing said it “fully” supports the FAA and its own customers in the regulator’s latest action.

Since the terrifying incident just over two weeks ago, some airlines have inspected the earlier-built Boeing planes and observed “findings with bolts” holding the door plugs in place, according to the FAA. The regulator did not specify what the findings were.

After recent inspections of the newer Max 9s, United Airlines and Alaska Airlines have also both found loose bolts.

United, Delta Airlines and Alaska Airlines told CNN Monday that they had already started inspections of their Boeing 737-900ER aircraft and did not anticipate any disruptions to operations. “We have had no findings to date,” Alaska Airlines said.

According to data on Boeing’s website, it has delivered a total of around 500 of these airplanes to carriers that also include Turkish Airlines.

Alaska said in a statement it began inspecting its 737-900ER planes several days ago and had “no findings” so far. It expects to complete the remainder of its 737-900ER inspections without pulling the planes from service.

The FAA notice does not ground the earlier generation of plane. Instead, it recommends that airlines inspect the door plugs and the bolts “as soon as possible.”

The newer Max 9 aircraft continue to be grounded in the United States. The agency said it continued to review data collected from inspections of 40 sample aircraft as it considered how to determine if the planes were safe to fly again.

Olesya Dmitracova contributed reporting.