The Department of Transportation is investigating the Federal Aviation Administration's approval of Boeing's 737 MAX planes, people with knowledge of the matter told The Wall Street Journal on Sunday.

The investigation is looking at an anti-stall safety system suspected of playing a role in the October crash of Lion Air Flight 610, which killed 189 people. Investigators are working to determine if this same system was behind last week's crash of an Ethiopian Airlines jet shortly after take-off from Addis Ababa; that accident left 157 people dead. On Sunday, Ethiopia's transport minister said there were "clear similarities" between the two crashes.

The inquiry was launched following the Lion Air crash, the Journal reports, and is being conducted by the department's inspector general. Investigators are trying to figure out if the FAA used the correct design standards and engineering analyses when certifying the 737 MAX's anti-stall system, called the maneuvering characteristics augmentation system. It's meant to help pilots should the plane's nose suddenly go up more than expected, and during tests, Boeing determined that the feature could assist crews during lower-altitude stalls, the Journal reports. Experts say the risks that came along with this system were underestimated and not explained in manuals or during pilot training.

The inquiry is focusing on two FAA offices in the Seattle area: one that certifies the safety of new aircraft models and another that mandates training requirements and signs off on training programs, officials told the Journal. People at those offices have been told not to delete or tamper with documents and emails. Catherine Garcia