The Federal Aviation Administration said on Wednesday that an initial round of inspections of 40 Boeing 737 Max 9 planes had been completed, but that those aircraft and scores of other Max 9 planes would remain grounded as the agency finalized an inspection process for them.

On Friday, the F.A.A. announced that it was requiring the 40 inspections before it would approve new inspection and maintenance instructions developed by Boeing. The agency grounded 171 Max 9 planes this month after a door panel blew off an Alaska Airlines flight while it was ascending after taking off from Portland, Ore., forcing an emergency landing.

In its statement on Wednesday, the F.A.A. said it would review the data from the 40 inspections, and that the 737 Max 9 planes with the door panels would remain grounded until the agency signed off on the instructions for airlines to inspect the planes. The door panels go where an emergency exit door would in a different configuration of the aircraft.

“The safety of the flying public, not speed, will determine the timeline for returning these aircraft to service,” the agency said in the statement.

Last week, the F.A.A. announced it was investigating whether Boeing failed to ensure that the 737 Max 9 was safe and conformed to the design approved by the agency. The incident involving the Alaska Airlines flight did not result in any serious injuries, but it could have been far graver had it occurred when the plane was at its cruising altitude.

In its statement on Wednesday, the F.A.A. said it was “investigating Boeing’s manufacturing practices and production lines, including those involving subcontractor Spirit AeroSystems,” which produces the fuselage of the 737 Max.

Boeing declined to comment on the F.A.A.’s statement. The plane maker has said it will cooperate with the agency’s inquiry, and it announced on Monday that it would make changes to its quality control processes. The company’s chief executive, Dave Calhoun, also visited Spirit AeroSystems in Wichita, Kan., on Wednesday and took part in a town-hall meeting with employees there.

A spokesman for Spirit, Joe Buccino, said the company was “supporting Boeing’s efforts with the F.A.A. and the affected airlines as they inspect the 737-9 fleet and work to safely return those airplanes to service.”

After a closed-door briefing for members of the Senate Commerce Committee on Wednesday, Jennifer Homendy, the chairwoman of the National Transportation Safety Board, said the door panel that came off the plane had been manufactured by Spirit in Malaysia.

Ms. Homendy said the panel, also known as a door plug, had been transported to Spirit’s plant in Wichita, where the plane’s fuselage was built. The fuselage was then brought by rail to Boeing’s factory in Renton, Wash., Ms. Homendy said.

The safety board is investigating what caused the door plug to detach, and Ms. Homendy said the board’s inquiry would look at the plug’s entire journey.