U.S. regulators considered grounding some Boeing 737 MAX planes last year after learning of a problem with a system that is now the main suspect in two deadly crashes, a source close to the matter said.

Investigators in the Lion Air crash in October off the coast of Indonesia and the Ethiopia Airlines disaster in March have zeroed in on the planes' anti-stall system, called the Maneuvering Characteristics Augmentation System, or MCAS.

Last year, inspectors with the Federal Aviation Administration (FAA) discovered Boeing de-activated a signal designed to advise the cockpit crew of a malfunctioning of the MCAS system, the source said.

The inspectors were in charge of monitoring Southwest Airlines, the biggest user of 737 MAX planes, with a fleet of 34 of them at the time, added the source.

Before the Lion Air crash, which killed all 189 people on board, "the (signals) were depicted as operable by Boeing on all MAX aircraft" regardless of whether the cockpit crew thought they had them turned on or off, said a Southwest spokeswoman.

Read more: American Airlines cuts profit forecast as 737 MAX woes bite

She said after the accident, Boeing told Southwest the signals were "turned off unless they were specifically designated as being turned on" – prompting the airline to choose that option for all its aircraft.

It was at that point inspectors learned Boeing had opted to make the malfunction alert an optional extra costing more money – and had deactivated the signal on all 737 MAX delivered to Southwest without telling the carrier.

They considered recommending grounding the planes as they explored whether pilots flying the aircraft needed additional training about the alerts, said the source.

They decided against that – but never passed details of the discussions to higher-ranking officials in the FAA, the source said.

The FAA would not comment on the planes coming close to being grounded last year.

The Ethiopia Airlines crash left all 157 people on the plane dead and led to all Boeing 737 Max planes all over the world being grounded. In this case too the MCAS is being looked at as a possible cause of the crash.