(CNN) -- South African police lied about events in last year's Marikana miners' strike in which police fired on thousands of machete-bearing workers, an investigating commission said Thursday.
Thirty-four miners were killed in the incident.
The Marikana Commission based its announcement on newly discovered police documents and computer hard drives belonging to South African Police Services.
Police earlier claimed those documents did not exist, the commission said.
"We have obtained documents which in our opinion demonstrate that the SAPS version of the events at Marikana, as described in the SAPS presentation to this commission and in the evidence of SAPS witnesses at this commission, is in material respects not the truth," the commission said. "We do not make this statement lightly."
The body said police officials should have an opportunity to respond to the findings. "However, we have to say that absent a convincing explanation, the material which we have found has serious consequences for the further conduct of the work of this commission," it said.
Some documents create the illusion that they were prepared as events unfolded, but in fact they were "constructed after the events to which they refer," the commission said.
The deaths of the 34 miners in the August 2012 incident made the strike the bloodiest labor dispute in South Africa since the end of apartheid.
The shootings at the Marikana mine came after deaths earlier in the week, including those of two police officers who were hacked to death. Tensions were intense partly because of the presence of competing trade unions, the Association of Mineworkers and Construction Union and the National Union of Mineworkers.
The mine, about two hours northwest of Johannesburg, is operated by Lonmin, which is listed on both the London Stock Exchange and Johannesburg Stock Exchange, and is the world's third largest platinum producer. The bulk of its 28,000 employees work at the mine, and around 23% belong to the AMCU.
The violence has prompted some people to draw parallels with the country's days of apartheid rule, which ended in 1994.
CNN's Irene Chapple and Errol Barnett contributed to this report.