Seven people, including four children, were killed when an overloaded van overturned on an expressway in east China's Zhejiang Province Monday night, traffic police said Tuesday.

The van hit a median barrier and overturned at 10:20 p.m. Monday on the G50 Expressway, leaving 7 people dead as of 8 a.m. Tuesday, according to traffic police from the Zhejiang Provincial Public Security Department.

An initial investigation showed that at least 12 passengers were riding in the 6-seat van when it overturned, police said.

The parents of the children are mostly migrant workers who rented the van to bring their children back to them for summer vacation, police said.

A further investigation is under way.