Boeing just can’t seem to catch a break. The much-beleaguered Boeing 737 Max is the subject of new safety concerns, as debris has been found in the fuel tanks of several new planes in storage in Washington and Texas, waiting to be delivered to airlines.

A Boeing company spokesperson told the BBC, “While conducting maintenance we discovered Foreign Object Debris (FOD) in undelivered 737 Max airplanes currently in storage. That finding led to a robust internal investigation and immediate corrective actions in our production system.”

FOD is defined by the Federal Aviation Administration as “any object, live or not, located in an inappropriate location in the airport environment that has the capacity to injure airport or air carrier personnel and damage aircraft.”

The entire fleet of new but undelivered aircrafts, which amounts to 400 planes, is being inspected for FOD.

In a message to Boeing employees, Mark Jenks, vice president and general manager of the 737 program, said, “FOD is absolutely unacceptable. One escape is one too many.”

All 737 Max airplanes have been grounded by regulators since March 2019, after two deadly crashes believed to be linked to software malfunctions.

While a thorough investigation of the issue is underway, Boeing has insisted that it will not delay the 737 Max’s return to service. And indeed, the model is slated to be back in the skies by mid-2020.