The Federal Aviation Administration may be expanding its investigation into suspected structural problems found in a small portion of American Airlines jets, according to Wall Street Journal.

At least one of the Fort Worth-based airline’s McDonnell Douglas MD-80 series jets was believed to be in such poor shape that it was ferried out without passengers to a maintenance base at low altitudes to avoid stressing the fuselage by pressurizing it. It cited unnamed people familiar with the matter.

But American Airlines spokesman Tim Wagner told the newspaper the carrier has responded to the agency’s formal “letter of investigation”. According to Wagner, any actions or precautions taken this year “whether airplane movements or repairs, were done with the highest level of safety in mind”.

Last month, an FAA official said regulators were investigating repairs to the rear bulkhead of aircraft in the aging fleet.

The Journal also reported Saturday that preliminary FAA findings showed as many as 16 jets that were operated for months despite substandard repairs.