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Quality control troubles continued to mount for Boeing after potentially damaging debris was discovered inside wing fuel tanks of several undelivered 737 Max airplanes during routine maintenance.

The airplane manufacturer has ordered a "robust internal investigation" of its entire 737 Max fleet, consisting of 400 airplanes that are in storage to rectify production concerns, the company said Wednesday.

Foreign object debris, or FOD, was found in the fuel tanks of several 737 Max planes at storage facilities in Washington and Texas, although it is unclear exactly how many planes were affected, according to the aviation blog Leeham News, which first reported the issue.

Foreign object debris, which includes items like tools or rags left behind by workers during assembly or maintenance, reflects a quality control issue on the assembly line. Leftover debris can cause short-circuiting or other mechanical problems on airplanes.

"FOD is absolutely unacceptable. One escape is one too many," Mark Jenks, vice president and general manager of the 737 program and Boeing's site in Renton, Washington, wrote in a message to all 737 employees. "During these challenging times, our customers and the flying public are counting on us to do our best work each and every day."

The recent setback is the latest in a string of problems for the company, which has been dogged by safety concerns regarding its troubled 737 Max airplanes.

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The Federal Aviation Administration grounded the 737 Max on March 13, three days after an Ethiopian Airlines flight crashed en route to Kenya, killing all 157 people on board.

Another 737 Max, Lion Air Flight 610, crashed off Indonesia in October 2018, killing 189 people.

Boeing has said automated anti-stall software contributed to the crashes. The debris problem is not related to the technical issues that grounded the planes.

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The company said it has already held a series of meetings on the factory floor to share a new process to stop debris, including "updated instructions and required checklists for teammates working in the fuel cell areas, additional verifications including inspections, audits and checks into our tank closure process to ensure there is zero FOD within the fuel tanks, and new signage added in these work areas to help remind teammates of the appropriate steps to take."

The FAA said in a statement that it was aware of the issue and that it had "increased its surveillance based on initial inspection reports and will take further action based on the findings."

Inspections may take up to three days for each airplane, because fuel has to be drained and vapors must be dissipated before inspectors can check tanks, Leeham reported.

Boeing cut almost 1,000 quality inspector jobs over two years to overhaul its manufacturing procedures, leading inspectors to voice grave concerns over quality control, according to The Seattle Times.

This is not the first time debris has shown up in Boeing planes — damaging debris has been a chronic issue for the airline manufacturer.

Debris was found in 787 Dreamliners and in KC-46A refueling tankers, which were twice rejected by the Air Force because of the problem, according to The Times. Eight tools were found in aircraft delivered to the military and two more in tankers delivered to the Air Force.

An investigation by the The New York Times shortly after the Ethiopian Airlines crash unearthed countless manufacturing defects, safety errors and complaint cover-ups in Boeing's South Carolina factory.

Since the grounding, Boeing has been under tremendous pressure to alleviate and recertify the 737 Max airplanes.

Despite the debris issue, the company says it remains optimistic and insists that the debris inspections will not affect the estimated timeline for a return to service, which could be as early as this summer.