A motor vehicle accident which killed 15 people and injured one on Saturday in Boli county of Heilongjiang province was caused by serious overloading, excessive speed and a tractor carrying passengers illegally, according to the Heilongjiang Provincial Department of Public Security on Monday.

The accident happened around 4 am on Saturday on a section of the G229 national highway.

A heavy truck running from west to east ran into the back of a tractor towing a trailer, killing the driver of the truck and 14 people on the tractor and trailer instantly.

The heavy truck is suspected of serious overloading and excessive speeds, the department said, adding the driver didn't keep a safe distance from the tractor in front, which is the direct cause of the accident.

The driver of the tractor was driving without a license and the tractor was carrying passengers illegally, which aggravated the consequences of the accident, according to the department.