The National Highway Traffic Safety Administration has launched an investigation to determine how quickly car rental companies fix recalled vehicles.

NHTSA said it will review nearly 3 million vehicles manufactured by General Motors, Chrysler and Ford that were sold to car rental companies. The agency said the investigation will give "an indication of how completely and how quickly rental car fleets, in general or individually, perform necessary recall-related repairs or other remedies on the vehicles owned and then leased for use on the roadways."

NHTSA said the investigation was prompted by "allegations of personal injury and death claimed to have been caused by safety defects and failures to conform to minimum Federal Motor Vehicles Safety Standards on rental car vehicles..." NHTSA also noted a petition was filed with the Federal Trade Commission to stop one car rental company from renting unrepaired recalled vehicles.

The Center for Car Safety and Consumers for Auto Reliability and Safety filed that petition with the FTC in August to stop Enterprise and its parent company from renting out unrepaired recalled vehicles to customers.

The FTC petition followed a lawsuit filed by Chuck and Carol Houck against Enterprise Rent-A-Car after the 2004 deaths of their daughters, Raechel and Jacqueline.

The two women rented a Chrysler PT Cruiser from Enterprise in California. The vehicle was recalled by the manufacturer for a power steering hose defect that was known to cause an underhood fire, but was never repaired.

The two women were unaware of the safety recall. They died in a head-on traffic collision with an 18-wheeler on the 101 north highway near King City, Calif., on Oct. 2, 2004.

A jury awarded their parents $15 million on June 9, 2010 in a lawsuit against Enterprise Rent A Car.

Enterprise spokeswoman Laura Bryant told the Detroit News the company would cooperate with NHTSA in any inquiry involving current practices.

"If and when manufacturers recommend that vehicle owners park or ground their vehicles, Enterprise promptly does so," she said. "In most cases, we place a 'hold' on recalled vehicles so they are not rented until the recall work is completed."

No federal law requires rental car companies to fix vehicles before they return to service, Sharon Faulkner, executive director of the American Car Rental Association, told the Detroit News. Most companies quickly repair vehicles once they get a notice, she said.

"You pull those cars and you park them," Faulkner said. "It's just foolish for anyone to risk a lawsuit, death or injury.

"It can be months before a recall notice gets to a rental company," she said, adding that automakers could work to better notify companies.