Multiple attackers killed 12 people and wounded at least nine in a late-night mass shooting in Johannesburg, police said Wednesday, the latest in such attacks that have shaken the South African city.

Police said they believe more than 10 suspects were driven in a minibus to an informal settlement in the Cleveland suburb of Johannesburg late on Tuesday night and once there, opened fire on people.

South Africa has seen several high-profile mass shootings recently, including two in December that killed more than 20 people. One of the attacks had also involved multiple shooters.

The shootings are sometimes linked to illegal mining gangs that operate in and around Johannesburg. The suburb of Cleveland is connected to illegal mining activity.

According to a police statement, the attackers “moved through the area, opening fire on residents and community members at multiple locations before fleeing the scene in the same vehicle.”

The victims were nine men and three women, according to police. Eleven died at the scene and another victim died in the hospital.

Police were searching for the attackers but no arrests had been made. The motive for the shooting remained unclear, they said.

Informal settlements in South Africa are unplanned residential areas usually made up of shacks or similar structures.

“Provincial and district detectives, supported by crime intelligence and forensic experts, have been mobilized to investigate the incident and track down the suspects,” said police spokesperson Athlenda Mathe.

Police commissioner Tommy Mthombeni said the attackers were “heartless” but refused to link the killings to illegal mining until the investigation was complete.

“We are still investigating, but what we have seen here is a criminal act. We have deployed all required units,” he said, adding that crime intelligence officials have also been deployed to the area.