WASHINGTON (AP) — Federal regulators are expanding an emergency order for airlines to inspect fan blades in the engines of Boeing 737 jets for cracks that indicate wear and tear.

The Federal Aviation Administration said Tuesday it will require blades on certain CFM engines be inspected before they hit 20,000 flights. Engines that have reached that milestone must be inspected by around late August. Follow-up checks will be required every 3,000 flights — 18 to 24 months.

Last month the FAA required emergency inspections of blades in engines after 30,000 flights.

Investigators believe cracks caused a fan blade on a Southwest Airlines jet to shear off last month, starting an engine breakup that left one passenger dead after debris hit the plane.

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