The National Highway Traffic Safety Administration (NHTSA) has upgraded its probe involving 65,347 Kia Sorento SUVs from a preliminary evaluation to an engineering analysis of shattering sunroofs.

Reuters reported the investigation had been upgraded following 26 consumer complaints about the shattering of sunroof glass in the SUVs from model years 2011 to 2013.

Of the 26 instances, 24 occurred while the vehicle was moving, and in four cases, the passengers were hurt by the shattered glass.

NHTSA said, "Kia stated in its response that their field inspections identified only one potential cause for the sunroof breakage - external impacts from rocks or other foreign objects encountered while the vehicle was in motion."