A head-on collision between a truck and a bus ferrying workers to a mine in northern South Africa’s Limpopo province has killed 20 people, authorities say.

The construction company Murray & Roberts Cementation said on Monday that 17 of the dead were its workers who were being transported to the Venetia mine in Musina close to the Zimbabwe border. Four workers were injured in Sunday’s accident.

The company said it was offering support to the families of the dead. It is still unclear who the other three victims are.

The workers were reportedly going to work on an underground project at the mine, which is one of the biggest diamond mines in the country and is owned by the mining giant De Beers. It accounts for more than 40 percent of the country’s annual diamond production and employs more than 4,300 staff, including many locals.

South Africa has one of the most developed road networks on the continent but also has one of the worst road safety records.

Traffic officials were still on the scene of the accident on Monday to investigate the cause.

Provincial Transport Minister Florence Radzilani, who visited the crash site on Monday, lamented the state of some of the trucks that travel on the roads.

“We always talk to our law enforcement officers and send out messages to these drivers to say they must make sure the vehicle is roadworthy, so that you don’t put the lives of innocent people at risk,” she said.

Radzilani said some truckers avoid roads where they know traffic officials patrol because their vehicles are not roadworthy or the drivers do not have valid permits.

“We are waiting on investigators, but there are already allegations about the truck driver not having a license and the truck not being roadworthy,” she told public broadcaster SABC.