DALLAS — Southwest Airlines Co., which is accused of operating planes that had missed key safety inspections, said Tuesday it has placed three employees on leave and hired an outside expert to review its maintenance procedures.

The airline also said it has promised federal regulators that it will fix any shortcomings in its system of tracking maintenance work.

Last week, the Federal Aviation Administration proposed a $10.2 million civil penalty — the largest ever against an airline — after finding that Southwest had missed safety inspections for dozens of planes, then kept flying some of them before they could be examined.

The airline hired JDA Aviation Technology Solutions, a Washington-based firm headed by a former longtime FAA senior official.

Southwest did not identify the employees who were placed on leave by name or position. The airline said it was cooperating with the investigation. Spokeswoman Beth Harbin said the three are being paid.