The U.S. auto safety regulator said today it was opening a preliminary evaluation into 210,960 Ford vehicles following complaints alleging diesel fuel leaks that may result in a fire.

The National Highway Traffic Safety Administration said its Office of Defects Investigation has received 27 complaints, including 12 reports of fires and a total of four injuries, and that it would investigate Ford F-250, 350, 450, and 550 Super Duty vehicles equipped with 6.7L diesel engines of model years 2015-2021.

The agency said a fracture in the secondary fuel filter in the vehicles could result in a fire, as the proximity of the filter to high heat sources creates a potential for auto-ignition thermal events and fires.

The NHTSA has contacted the filter’s maker, Allevard Sogefi USA, and said it learned the secondary fuel filter is tested to operate at reduced levels of pressure during the manufacturing process, compared with the possible maximum operating pressure in the fuel system of the vehicle.

Ford and Allevard Sogefi USA’s parent Sogefi were not immediately available for comment.

The agency’s preliminary evaluation is the first step to determine whether the vehicles pose an unreasonable risk to safety. It could close the investigation without taking any potential action.

Separately, the NHTSA said on Thursday it had “significant safety concerns” over Ford’s recall of more than 42,000 SUVs over concerns on fuel leaks that could lead to an engine fire.

While Ford had proposed an engine control software update and installation of a drain, the NHTSA said the company’s plan “does not address the root cause of the issue and does not proactively call for the replacement of defective fuel injectors prior to their failure.”