The Federal Aviation Administration said on Sunday that a joint investigation conducted with Boeing found over 300 planes worldwide may include parts that may have been improperly manufactured by a Boeing supplier. As many as 148 parts may be affected.

Two models — the 737 Max and 737 NG — may have faulty “leading edge slat tracks,” a wing part important during takeoff and landing. The FAA warned in a statement that these parts “may be susceptible to premature failure or cracks.” While a failed leading edge slat track would not cause a complete loss of the aircraft, it could still cause damage to an aircraft in flight, according to the agency.

The investigation found 33 737 Max planes with affected parts, in addition to 32 affected 737 NG planes, are in the U.S.

The results of this investigation are the latest hit to Boeing 737 Max aircrafts, which were grounded worldwide months ago after two deadly crashes that together killed over 300 people. In the U.S., American Airlines, United Airlines and Southwest Airlines all include 737 Max planes in their fleets.

Consumer advocate Ralph Nader called in April for consumers to boycott the model.

“Those planes should never fly again,” said Nader, whose niece was among the 157 victims of the March 2019 crash. “Boeing executives pushed its engineers to press an aging design beyond its limits.”

The FAA said it will issue an Airworthiness Directive, mandating that airlines with affected planes remove and replace faulty parts within 10 days.