They are also building the profile of a man who warned a pregnant woman at the mall to flee for her own safety moments before Saturday's attack, he said.
Investigators are tracing the car's ownership after it was retrieved outside the mall, the official said on condition of anonymity because he was not allowed to reveal such details while the investigation is ongoing.
They are looking at more vehicles that may have been used by the attackers, he said, but gave no more details.
The car's trunk was open as the investigators took pictures and notes, but it was impossible to tell what exactly they were seeing.
Kenyan police have given little information since the attack that shocked this East African nation, saying the investigation has only just began into the storming of the mall on Saturday by Islamic militants throwing grenades and using assault rifles.