NEW YORK/WASHINGTON (Reuters) - Federal investigators were assigned last year to monitor Southwest Airlines' LUV.N maintenance operations, the Federal Aviation Administration said on Friday, after whistleblower complaints of mistreatment of mechanics raised safety concerns.

FILE PHOTO: A Southwest Airlines plane is seen at Los Angeles International Airport (LAX) in the Greater Los Angeles Area, California, U.S., April 10, 2017. REUTERS/Lucy Nicholson/File Photo

The agency found no rule violations, it said, and the assignment of additional inspectors was standard practice to preclude any deterioration in maintenance.

The FAA issued a statement on the incident as investigations continue on last month’s midair engine fan blade blowout on a Southwest plane that killed a passenger. The two incidents are unrelated.

The FAA, which oversees U.S. civil aviation, investigated Southwest in 2017 after employee reports that airline management had discouraged maintenance workers from reporting mechanical issues.

The investigation at the time found that some management practices were being used to “influence a relaxing of standards, to look the other way, or to gain a degree of approval through a leniency of standards.” The agency said the additional scrutiny ended at an unspecified date.

In response to inquiries regarding the matter on Friday, the FAA said: “The Federal Aviation Administration investigated the whistleblower complaints against Southwest Airlines and found no violations of Federal Aviation Regulations.

“However, investigators raised concerns that the strained relationship between the company and mechanics might potentially affect safety. As a precaution, the FAA assigned additional inspectors to monitor the company’s maintenance operations. This is standard protocol during contentious labor negotiations and other situations that warrant increased scrutiny.”

Southwest and its mechanics had been locked in acrimonious contract negotiations for the last five years. In April, they reached an agreement in principle toward formalizing a five-year deal.

In a statement on Friday, Southwest pushed back against the FAA findings.

“Southwest’s Culture of Safety does not align with the comments made by the individual FAA employee who conducted the investigation. Nothing is more important in our business than Safety and, as such, compliance is always our theme,” the carrier said.

A representative for the Aircraft Mechanics Fraternal Association, Bret Oestreich, said: “The culture of safety in airlines has really deteriorated. The airlines are more concerned about the stockholders and the on-time performance over safety.”