F.A.A. Investigating Whether Boeing 737 Max 9 Conformed to Approved Design

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

The F.A.A. said the investigation stemmed from the loss of a fuselage panel of a Boeing 737 Max 9 operated by Alaska Airlines shortly after it took off on Friday from Portland, Ore., leaving a hole in the side of the passenger cabin. The plane returned to Portland for an emergency landing.

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

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

The National Transportation Safety Board is investigating why the panel, also known as a door plug, flew off the Boeing jet. The safety board is trying to determine whether bolts that would have kept the panel in place were missing or were installed incorrectly. The plug is placed where an emergency exit would be if the plane had the maximum number of seats possible.

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

“Boeing’s manufacturing practices need to comply with the high safety standards they’re legally