Boeing needs to pay more heed to how pilots react to emergencies in its safety assessment of the 737 Max plane, US transport chiefs have said.
The 737 Max has been grounded since March following two fatal crashes.
The US Federal Aviation Administration (FAA) must decide if the plane is safe.
But according to the US National Transportation Safety Board (NTSB), the crews in the fatal crashes "did not react in the ways Boeing and the FAA assumed they would".
The 737 Max plane has not flown commercially since an Ethiopian Airlines aircraft crashed shortly after take-off on 10 March, killing 157.
It followed a Lion Air crash on 28 October last year which killed 189.
In both incidents, investigators have focused on the role played by a software system called MCAS (Manoeuvring Characteristics Augmentation System), which was designed to make the aircraft easier to fly.
Inquiries have shown the software - and the failure of sensors - contributed to pilots not being able to control the aircraft.
Boeing has said it is revising the plane's software to improve safeguards.
But the director of the NTSB's Office of Aviation Safety, Dana Schulze, said Boeing "did not look at all potential flight deck alerts and indications that pilots might face when this specific failure condition occurred in Lion Air and Ethiopian Airlines".
Earlier this month, Europe's aviation safety watchdog, the European Aviation Safety Agency (Easa), said it would not accept a US verdict on whether the 737 Max was safe.
Instead, it will run its own tests on the plane before approving a return to commercial flights.