The Federal Aviation Administration said Thursday it had opened an investigation into whether Boeing failed to ensure its 737 Max 9 plane was safe and built to match the agency’s approved design.

The FAA said the investigation was prompted by the loss of a fuselage panel of a Boeing 737 Max 9 operated by Alaska Airlines shortly after takeoff Friday from Portland, Oregon, leaving a hole in the side of the passenger cabin. The plane returned to Portland to make an emergency landing.

“This incident should never have happened and cannot happen again,” the agency said.

In a letter to Boeing dated Jan. 10, the FAA said that after the Portland incident, it was notified of additional problems with other Boeing 737 Max 9 aircraft. The letter does not detail what other problems were reported to the agency. Alaska and United Airlines, which operate most of the Max 9s in use in the United States, said Monday they discovered loose panel hardware while conducting preliminary inspections on their planes.

The new investigation is the latest setback for Boeing, which is one of only two suppliers of large planes to most airlines. The company has struggled to regain public trust after two crashes, in Indonesia in 2018 and Ethiopia in 2019, in which the 737 Max 8 killed a total of 346 people.

The National Transportation Safety Board is investigating why the 737 Max 9’s panel, also known as the door plug, flew off the Boeing plane. The safety board is trying to determine if bolts that would have prevented the panel from moving and opening were missing or improperly installed. The plug is placed where an emergency exit would be if the plane had the maximum number of seats possible.

Before Thursday’s announcement, the FAA had been working with Boeing to review the company’s instructions for inspecting grounded 737 Max 9 planes. The announcement of the review came after reports of loose bolts from two airlines.

“Boeing’s manufacturing practices must meet the high safety standards they are legally required to meet,” the FAA said in the statement announcing the investigation.

Boeing CEO Dave Calhoun on Tuesday promised transparency in the company’s response to the incident. He also said the company was “acknowledging our mistake” without explaining what he meant. Boeing has declined to elaborate on that comment.

“We will fully and transparently cooperate with the FAA and NTSB in their investigations,” Boeing said in a statement.

United has 79 planes and Alaska has 65, but Alaska has had the highest share of cancellations due to grounding because the Max 9 makes up 20 percent of its fleet.

Mark Lindquist, an attorney who represented the families of victims involved in the Max 8 crashes, said the FAA was being more proactive than in the past by quickly opening an investigation into the Boeing 737 Max 9. Lindquist said the FAA would analyze the plane much more broadly than the NTSB, whose goal is to establish the cause of accidents and make recommendations on how to prevent them.

“The tone of this announcement indicates that the FAA believes there was a potential for loss of life and the severity of Boeing’s quality control problems,” Lindquist said.