Boeing Co. ’s 737 MAX planes are unlikely to be ready to carry passengers again until 2020 because of the time it will take to fix flight-control software and complete other steps, an increasing number of government and industry officials say, even as the company strives to get its jet back into service this year.

The situation remains fluid, no firm timeline has been established and Boeing still has to satisfy U.S. regulators that it has answered all outstanding safety questions. But under the latest scenario, the global MAX fleet is now anticipated to return to the air in January 2020, a full 12 months after the plane maker proposed its initial replacement of software eventually implicated in a pair of fatal crashes—one in October and one in March—according to some Federal Aviation Administration officials and pilot-union leaders.

The process of developing and certifying revised software and pilot-training changes has been repeatedly delayed, with airlines scrambling to cope with slips month after month.

Boeing executives, FAA engineers and international aviation regulators have steadily expanded their safety analyses to cover a growing list of issues spanning everything from emergency recovery procedures to potentially suspect electronic components. Some of those assessments are further complicated because they also cover earlier 737 models.

Already, carriers have given up on flying their MAX planes until late this year. American Airlines Group Inc. said Sunday that it would keep the plane off its schedules through Nov. 2, two months beyond its previous target of an early September return. It is the fifth time American has pushed off MAX flying since it first had to call off flights when regulators grounded the plane in March.