WASHINGTON — The Federal Aviation Administration says Boeing's grounded 737 Max 9 jets can begin flying again after a "thorough inspection and maintenance process." But the agency also imposed sweeping jet production restrictions at the company's factories.

It's been nearly three weeks since federal regulators took 171 Boeing aircraft out of service after part of the fuselage of an Alaska Airlines jet blew out at 16,000 feet after departing the Portland International Airport.

"We grounded the Boeing 737-9 MAX within hours of the incident over Portland and made clear this aircraft would not go back into service until it was safe," FAA Administrator Mike Whitaker said in a statement late Wednesday. "The exhaustive, enhanced review our team completed after several weeks of information gathering gives me and the FAA confidence to proceed to the inspection and maintenance phase."

The FAA's announcement comes amid growing questions about quality control at the Boeing factory where the 737 was assembled.

An apparent Boeing whistleblower says that mistakes inside the aerospace giant's plant in Renton, Wash. were likely to blame for the incident. The self-described Boeing employee alleges that four key bolts that are supposed to hold the door in place were never reinstalled after maintenance work before the jet left the factory.

The FAA announcement did not mention those allegations. But the agency has its own concerns about Boeing's production and manufacturing processes.

"This won't be back to business as usual for Boeing," Whitaker said, announcing that the FAA would not grant any requests from Boeing to expand production of the Max aircraft, "until we are satisfied that the quality control issues uncovered during this process are resolved."

The production cap applies to Max 8 and 9 (which are in use around the world) as well as the upcoming smaller Max 7 and larger Max 10 variants. Critics say Boeing has been rushing production to clear a lengthy backlog of orders following a previous grounding of the Max aircraft following a pair of fatal crashes in 2018 and 2019 which killed a total of 346 people. Those crashes were blamed on a faulty flight control system on the new planes.

Boeing declined to comment on the whistleblower allegations, citing an ongoing investigation by the National Transportation Safety Board. Investigators at the NTSB had previously raised the possibility that the bolts on the door plug panel were not properly installed.

Following the FAA's announcement, Boeing said it would work with regulators and airlines to get the grounded planes back in the air.

"We will continue to cooperate fully and transparently with the FAA and follow their direction as we take action to strengthen safety and quality at Boeing," said a statement from Boeing spokesperson Jessica Kowal. "We will also work closely with our airline customers as they complete the required inspection procedures to safely return their 737-9 airplanes to service."

United and Alaska Airlines have both been forced to cancel thousands of flights while waiting for this final inspection guidance from regulators and Boeing. On Tuesday, the CEOs of both companies were sharply critical of Boeing in separate interviews.

In a letter to United employees on Wednesday, chief operating officer Toby Enqvist said the company would begin the process of inspecting its fleet of 79 grounded jets.

"We are preparing aircraft to return to scheduled service beginning on Sunday," Enqvist said. "We will only return each MAX 9 aircraft to service once this thorough inspection process is complete."