A hacker has attacked a South African police website, downloading the details of whistle-blowers who reported crime, an official has said.
An "oversight" led to the data being kept on an insecure site, he added.
The anonymous hacker said the attack was over the failure so far to prosecute policemen who shot dead 34 miners in Marikana last August.
Nearly 16,000 whistle-blowers had their details published online after the cyber-attack, local media reports.
South Africa has one of the highest crime rates in the world, and police have been under pressure to build better relations with the public in order to hunt down criminals, correspondents say.
State Information Technology Agency (Sita) official Daniel Mashao said the hacker had accessed about "15,000 lines" posted anonymously by members of the public.
"We are concerned because there is information where people have given further details to say 'I know about such and such [a crime],'" he said, the South African Press Association (Sapa) reports.
The data had been collected on a public server, Mr Mashao added.
"There was a little bit of an oversight, because it should have been removed afterwards," he is quoted as saying.
In a tweet on Friday, hacker "DomainerAnon" condemned the fact that no-one had been charged with the killing of the striking workers - the most deadly police action in South Africa since the advent of democracy in 1994.
"To date no officers have been brought to justice... This situation will not be tolerated," the hacker tweeted.
The information obtained by the hacker had already been in the public domain, senior police officer Lt Gen Bonginkosi Ngubane said, Sapa reports.
Nevertheless, police have closed the "leak" on their website, he said.
"I really do not believe that the situation has gone out of hand. The information will have to be withdrawn from the sites where it was illegally posted as soon as possible," he is quoted as saying.
A judge-led inquiry is presently investigating the August 2012 violence at the Marikana mine, owned by Lonmin, in South Africa's North West province.
The shooting caused national outrage, with many people accusing the police of using excessive force to break the protest.
Police say they opened fire in self-defence after being confronted by a group of miners, some of whom were armed.