Boeing’s (NYSE:BA) 737 Max 9 jets are facing continued grounding by U.S. aviation authorities, who have introduced additional requirements for the aviation giant following a midair emergency on an Alaska Airlines flight on January 5. This move highlights a more stringent stance by the federal government in response to safety concerns.

The U.S. Federal Aviation Administration has requested more information from Boeing (BA) before granting approval for inspection and maintenance procedures necessary for the resumption of flights. The agency aims to review data from initial checks of the door plugs on 40 planes using Boeing’s (BA) instructions before extending the verification to all grounded jets.

Commenting on the matter, FAA Administrator Mike Whitaker emphasized, “We are working to make sure nothing like this happens again. Our only concern is the safety of American travelers, and the Boeing 737-9 Max will not return to the skies until we are entirely satisfied it is safe.”

The agency initially grounded approximately 171 Boeing 737 Max 9 planes after an incident on Alaska Airlines Flight 1282, where a mid-cabin exit door plug was lost mid-flight, leading to an emergency landing at Portland International Airport in Oregon. The grounding order specifically applies to 737 Max 9 planes configured with a door plug in place of an emergency exit.

In light of the incident, the FAA has increased oversight of Boeing’s (BA) production and manufacturing, and has launched an investigation to ascertain whether the company failed to ensure that its planes were manufactured according to approved designs for safe flights.

Major carriers, including United Airlines (UAL) and Alaska Airlines (ALK), which operate versions of the grounded 737 Max 9, have been compelled to cancel hundreds of flights as part of the safety inspection procedures.