U.S. safety regulators have opened a probe of an estimated 938,000 Ford Motor Co mid-sized sedans due to complaints about steering issues, reported Reuters.

The National Highway Traffic Safety Administration initiated what is called a preliminary evaluation into certain models of the Ford Fusion and Fusion Hybrid, Lincoln MKZ and MKZ Hybrid and Mercury Milan from model years 2010 to 2012, according to a document on the safety agency's website.

A Ford spokeswoman said on Monday that the No. 2 U.S. automaker is cooperating in the probe.

A preliminary investigation is the first step in a process that could lead to a recall if regulators determine that a manufacturer needs to address a safety issue.

The NHTSA said it has identified 508 complaints alleging loss of steering power and increased steering effort in the models equipped with rack mounted electric power assisted steering. Four of the complaints allege the steering-assist failure resulted in increased steering effort that contributed to a loss of vehicle control and a crash, but no injuries were listed.

In addition, the NHTSA said it has identified related information in Early Warning Reporting field report data submitted by Ford.

Many of the complaints indicated observing a power steering warning message as the failure occurred and in some cases the condition was corrected by restarting the vehicle, according to the NHTSA. However, many reports said the condition returned again after restart.