The National Highway Traffic Safety Administration on July 29 opened an investigation into an estimated 4,900 Jeep Wranglers, manufactured by Fiat Chrysler Automobiles NV, over complaints of defective seat belt buckles.

The NHTSA's Office of Defects Investigation, or ODI, will probe 2015-2016 right-hand drive Jeep Wranglers that are commonly used for postal delivery in rural areas.

The investigation was triggered after the agency received three reports alleging that the driver's side seat belt buckle separated from the bracket that secures it to the seat frame. The issue makes the seat belt nonfunctional, according to the document.

"ODI is opening this investigation to assess the scope, frequency and consequence of this alleged defect in the subject," the office said.

There have been no reports of injuries or fatalities linked to the defect.