The body of the girl was sent for postmortem and police is investigating the case from various angles, Ballabhgarh police station in-charge Yogvindra said.
According to the driver and bus conductor, the girl was accompanied by a woman, two men and two other children when they boarded the bus at Chandigarh. They had all purchased tickets to Delhi and got down there.
The two didn't know the girl was left behind. When they checked the bus after reaching their final destination at Ballabhgarh, they discovered the body and informed the police.