(CNN) Boeing's 737 Max jets will remain grounded for weeks after the Federal Aviation Administration said Monday afternoon that the plane maker continues to work on a software fix.

Boeing had said it would submit the fix to the FAA last week, and it had gathered hundreds of industry representatives at its Seattle-area facilities last Wednesday to demonstrate the software changes

But the FAA said Monday that the company concluded "additional work" is needed.

"The FAA expects to receive Boeing's final package of its software enhancement over the coming weeks for FAA approval," the agency said in a statement. "Time is needed for additional work by Boeing as the result of an ongoing review of the 737 Max Flight Control System to ensure that Boeing has identified and appropriately addressed all pertinent issues."

Boeing acknowledged the new timeline, saying in a statement that the software change would be "completed in the coming weeks." It did not say why the timeline had changed.

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