The cause of a blast that killed 20 people and injured 31 others on June 29 in south China’s Guangdong province has been identified, local authorities said Friday.

A tanker truck, which was carrying 54.2 tonnes of solvent oil, was rear-ended by another truck at 4:20 a.m. when it was illegally parked on the Yanjiang Expressway in the provincial capital Guangzhou, according to the city government.

Oil leaked from the tanker truck. It then ran down 10 drain pipes to a ditch under the expressway bridge. The 41 tonnes of leaked solvent oil started to vaporize into a flammable gas, triggering a serial explosion and setting a container yard and some nearby construction sheds ablaze, the government said.

The drivers of the two vehicles and others believed to be responsible for the accident are under investigation.

The families of 18 victims have received compensation from the government to date.

Four out of the 31 injured have been discharged from the hospital while 27 others remain in hospital.