Safety regulators in the U.S. have closed investigations into approximately 500,000 Ford Motors and 100,000 General Motors vehicles without seeking a recall.

The news was announced Tuesday by the National Highway Traffic Safety Administration.

The Fords involved are the 2004 to 2007 model year Ford Crown Victoria, Mercury Grand Marquis and Mercury Marauder models. The cars were under investigation since a heat shield could rust, dislodge, and cause the steering to jam.

U.S. investigators looked over at least 18 complaints about the problem and determined that incidents take place rarely and infrequently, only 1.6 vehicles per 100,000.

NHTSA said that six of the 10 reported complaints to the agency were from one police department, according to Reuters. One report of a rollover crash on a highway entrance ramp could not be verified.

The problem with the GM vehicles involves the model year 2014 Chevrolet Impala sedans. Federal regulators received two complaints of unintended autonomous braking that caused rear-end collisions.

The affected Impalas were rental vehicles, neither of which had cruise control systems that can brake in emergencies.

GM investigators think that the drivers in both incidents accidentally activated an electric park brake systems.

"The closing of this investigation does not constitute a finding by NHTSA that a safety related defect does not exist," NHTSA said for each of the moves.

GM did find an issue with the parking brake that could lock the brakes after a car is started. The Detroit automaker recalled close to 133,000 cars, including the Impala for that issue.

The news was announced a couple days after Arizona said it was suing GM on claims it hid safety defects. The state is looking for an estimated $3 billion from GM, the New York Times reported.

Arizona Attorney General Tom Horne said civil penalties could reach up to $10,000 per violation, according to Reuters. 

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