U.S. safety regulators broadened a more than two-year probe into the possibility of unintended acceleration in about 480,000 Ford Motor Co sedans when floor mats are unsecured or stacked.

The National Highway Traffic Safety Administration upgraded its investigation to an engineering analysis, the second step in a process that could lead to a safety recall, NHTSA said Friday.

NHTSA also added a model and nearly doubled the number of vehicles involved in the action.

When the probe was announced in 2010, just over 249,000 Ford Fusion and Mercury Milan sedans were involved. Now, the agency is examining the Lincoln MKZ as well as the Fusion and Milan, documents filed on NHTSA's website show.

NHTSA said unsecured floor mats may block the accelerator pedal from returning to idle. A "heel blocker" in the floor pan may lift an unsecured mat into contact with the pedal, according to the NHTSA documents.

No deaths or injuries are tied to the issue.

Ford is "disappointed" by NHTSA's decision, "particularly since the condition under investigation relates to improperly installed, unsecured or double stacked floor mats," Ford spokeswoman Susan Krusel said in an email.

She added that the No. 2 U.S. automaker urges its customers to properly install and secure the floor mats. The driver's mat carries a warning to drivers to avoid stacking the mats on top of one another, she said.

In 52 complaints reported to Ford and safety regulators, drivers reported difficulty braking and continued high engine power when trying to slow down. The problem usually happened after the driver accelerated to pass slower vehicles or to merge with fast-moving traffic.

After the incidents, drivers and service technicians reported that the floor mat interfered with their ability to slow down the car. They also noted that the all-weather floor mats made by Ford or third-parties were unsecured.

(Reporting By Deepa Seetharaman; Editing by Richard Chang)