The Federal Aviation Administration is planning to review hundreds of new Boeing 737 Max planes individually before they can be delivered to customers, adding a wrinkle into the already-delayed certification of the jetliners, grounded since mid-March after two fatal crashes.

In a letter to a Boeing safety and compliance executive on Tuesday, the FAA said that the “large number of new 737 Max airplanes currently in storage presents a number of challenges for airworthiness certification, production, and delivery which significantly exceed any that the Boeing system has previously experienced.”

Boeing executives have repeatedly said they expect regulators to sign off on the planes this quarter and said Nov. 11 deliveries could resume as early as this month, a forecast that investors applauded. But the FAA and administrator Steve Dickson continue to say that the agency has no set timeline to allow the planes to return to market, setting up a public tug-of-war between the manufacturer and the FAA.

“The FAA has determined that the public interest and safety in air commerce require that the FAA retain authority to issue airworthiness certificates and export certificates of airworthiness for all 737 Max airplanes,” said the FAA’s letter to Boeing. “The FAA will retain such authority until the agency is confident that, at a minimum, Boeing has fully functional quality control and verification processes in place; delivery processes are similarly functional and stable; and Boeing’s 737 Max compliance, design, and production processes meet all regulatory standards and conditions for delegation and ensure the safety of the public.”

Boeing didn’t say whether the change would alter its long-held projection that it would win regulator approval in the fourth quarter but a spokesman said: “We continue to work closely with the FAA on the safe return to service of the Max fleet.”

The FAA didn’t say how long the process would take but a spokesman said the agency has “enough inspectors in place to accommodate Boeing’s delivery capacity.”

The 737 Max is Boeing’s best-selling plane and the manufacturer halted deliveries and slashed production of the planes after the worldwide grounding in March. Boeing has stored more than 300 of the new planes in Washington state and Texas.

Nearly 350 people were killed in two 737 Max crashes — one in Indonesia in October 2018 and another in Ethiopia in March — prompting several investigations, including a criminal probe. Lawmakers have criticized the FAA for its original approval of the planes and questioned whether the agency handed over too much of the certification work to Boeing.

Boeing has developed a software fix for the planes after flight-control software that was erroneously activated repeatedly pushed the nose of the planes down before both crashes. The FAA needs to sign off on the changes, review a test flight and evaluate human factors in the plane’s safety before the planes can fly commercially again.