An aerial photo shows Boeing 737 MAX airplanes parked on the tarmac at the Boeing Factory in Renton, Washington, March 21, 2019.

The U.S. Office of Special Counsel sent letters to President Donald Trump and lawmakers outlining its findings, which stem from a whistleblower complaint about qualifications of FAA inspectors.

The investigation is adding to scrutiny of the agency that certified the Boeing 737 Max to fly in 2017. The planes have been grounded worldwide since mid-March after two fatal crashes within five months of one another claimed the lives of all 346 people on board.

Some of the Federal Aviation Administration inspectors who worked on training requirements for the troubled Boeing 737 Max planes and other aircraft were underqualified and the air safety agency misled lawmakers about it, federal investigators said Tuesday.

The FAA's Office of Audit and Evaluation in February said it found that 16 of 22 safety inspectors did not complete formal training while 11 of the 16 lacked flight-instructor certificates.

The FAA told a Senate panel in April that all Boeing 737 Max inspectors who worked on the Flight Standardization Board that sets pilot training and procedures were qualified, but the Special Counsel's office said its findings contradicted that.

"The FAA is entrusted with the critically important role of ensuring aircraft safety," Special Counsel Henry J. Kerner said in a release. "The FAA's failure to ensure safety inspector competency for these aircraft puts the flying public at risk."

The FAA said it was reviewing the letter.

"We remain confident in our representations to Congress and in the work of our aviation safety professionals," the FAA said in a statement. "Aviation safety is always our foremost priority, and we look forward to responding to the concerns that have been raised."

"All of the Aviation Safety Inspectors who participated in the evaluation of the Boeing 737 MAX were fully qualified for those activities," it said.