WASHINGTON - The National Highway Traffic Safety Administration said it has opened a probe into how quickly rental car companies have repairs performed after vehicles within their fleets are the subject of a safety recall. 

The agency, part of the Department of Transportation, said the investigation will involve nearly 3 million vehicles -- 29 models -- in rental car fleets. 

The probe's aim is to provide "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."