Police in central China's Hubei Province are investigating the deaths of two preschoolers who were left on a school bus for nearly eight hours.

The two girls, both four years old, were put on a school bus of Redbud Kindergarten in Jingzhou City Tuesday morning, the local public security bureau said in a press release Wednesday.

It said the school bus, a van with more than 10 seats, arrived at the kindergarten at 8:30 a.m. and the children were escorted by the driver and a teacher to their classrooms.

The driver, surnamed He, found Chen Qingxue and Peng Yiwei under the back seat when he returned to the van at 4 p.m., the document said.

The two children were rushed to a hospital, where they were declared dead, it said.

The exact cause of the girls' deaths is still under investigation. But the temperature in Jingzhou city topped 31 degrees Celsius on Tuesday.

The two girls had similar family background. Both were under care of grandparents as their parents were working as migrants in faraway cities.

Similar tragedies have been reported in several Chinese cities in recent years as private kindergartens and preschools mushroom.

On Aug. 29, a 3-year-old boy was found dead after being left on a school bus in the city of Sanya in the tropical Hainan Province for nine hours. He had dozed off when other kids were escorted off the bus.

On Aug. 2, a 3-year-old girl was suffocated to death after eight hours in a school bus in Anqing of the eastern Anhui Province.

At least four other preschoolers, aged from 2 to 5, died in similar tragedies between 2007 and 2010.