The logo of the Ford Motor Company is seen outside a car dealership in Cape Town, South Africa, October 18, 2017. REUTERS/Mike Hutchings

WASHINGTON (Reuters) - The National Highway Traffic Safety Administration (NHTSA) said Thursday it had opened a preliminary investigation into 54,400 Ford Motor Co F.N pickup trucks after receiving five complaints that tailgates had unexpectedly opened while the vehicles were in motion.

The agency said the investigation was looking into the issue in 2017 model year F-250 and F-350 Super Duty pickup trucks.

NHTSA will investigate to determine if the issue poses an unreasonable risk to safety and whether the agency should seek a recall.

In October 2017, Ford issued a technical service bulletin addressing the issue, and found water intrusion in the wire harness as the root cause of tailgate problems, NHTSA said.

A Ford spokeswoman said the company was cooperating with NHTSA in the investigation.