CHICAGO -- Chicago-based United Airlines (UA) announced Saturday that it has temporarily grounded all of its Boeing 737 MAX 9 aircraft as required by the Federal Aviation Administration (FAA).

"United has temporarily suspended service on all Boeing 737 MAX 9 aircraft to conduct inspections required by the FAA. We are working with the FAA to clarify the inspection process and the requirements for returning all MAX 9 aircraft to service," the airline said in a statement released on Saturday.

The UA grounding comes after a piece of the fuselage blew out on an Alaska Airlines Boeing 737 Max 9 at an elevation of 16,000 feet on Friday, leaving a large hole and decompressing the passenger cabin.

The plane, destined for Ontario, California, made an emergency landing back in Portland 20 minutes after taking off.

The National Transportation Safety Board and the FAA had begun to review the damaged aircraft. No information about a possible cause has been provided.

The FAA placed an emergency order on the 737 MAX model line on Saturday, grounding 171 of the planes on an international level.

The incident is the third major safety concern for the line. The other two incidents were deadly crashes shortly after its debut.

Boeing said in a statement issued Saturday that it is "working to gather more information," adding that its "technical team stands ready to support the investigation."

The UA has 79 Boeing 737 MAX 9 in its fleet, including about 33 that have already received the necessary inspection required by the FAA, according to local media.