US regulators considered grounding some Boeing 737 MAX planes last year after learning belatedly of a problem with a system that is now the main suspect in two deadly crashes, a source close to the matter said Sunday. Investigators probing the Lion Air crash in October 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 discovered that the manufacturer had de-activated a signal designed to advise the cockpit crew of a malfunctioning of the MCAS system, the source said. These 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, said the source.