"The investigation will seek to establish if the police action was proportional to the threat posed by the miners," spokesperson Moses Dlamini said on Friday.
"It is still [too] early in the investigation to establish the real facts around this tragedy," he said.
National police commissioner Riah Phiyega said on Friday, 34 people were killed at the shooting.
"The total death of the protesters currently stands at 34 with more than 78 injured," Phiyega told reporters.
Investigators were working with the criminal record centre and ballistics experts, and would focus on collecting all the relevant evidence to assist in the investigation.
The police ministry said on Friday that "over 30" people died, while the National Union of Mineworkers (NUM) said that 36 people were killed when police opened fire on protesters gathered at a hill near the mine on Thursday afternoon. Later, the national police commissioner, Riah Phiyegah, said 34 were injured and 78 injured.
Another 10 people – including two police officers, two security guards and three NUM shop stewards – were killed in separate incidents since the start of an illegal strike last Friday.
The strike was believed to be linked to rivalry between the NUM and Amcu over recognition agreements at the mine. Workers also wanted higher wages.
They claimed to be earning R4 000 a month, with those living outside the hostel earning an extra R1 000. Reported demands have included pay of R12 500 a month.
'Working around the clock'
The workers had based themselves on a hilltop near the mine. Many of them were armed. They were shot when police tried to disperse the group.
A South African Press Association reporter on the scene said that after about three minutes of shooting, he counted 18 bodies lying on the ground.
Dlamini said IPID executive director Francois Beukman and a team from its national office would be briefed by an investigation team deployed at Marikana. He would then visit the scene.
Investigators from the IPID's offices in North West and Gauteng had been "working around the clock" since the shooting, he said.
Meanwhile, more then 50 police vehicles and at least 60 police officers were visible at the mine on Friday, ahead of a police briefing on the shootings. Two helicopters were intermittently circling the area.
"Be careful," said a security guard, who was busy checking in local and international media gathering for a press conference led by Phiyega.
Barbed wire added an extra layer of protection at the mine, which has been shut down for all except essential services.
Marikana deaths make world headlines
The incident has made international headlines, with the Guardian, New York Times, Sky News, Denver Post, BBC, Washington Post, CNN, and several other media reporting on it.
Wikipedia put up a detailed one-page entry on it with references to local newspapers and reporters.
On Thursday, police opened fire with live ammunition on armed, striking workers who had gathered on a hill in the area.
Ten people, among them police and security guards, had already died since the start of their protest a week ago.
The protests were believed to be linked to rivalry between NUM and Amcu over recognition agreements at the mine.
Workers also wanted higher wages. They claimed to be earning R4 000 a month, while those living outside the hostel were earning an extra R1 000. Reported demands included pay of R12 500 a month.
People around the world were using the social networking sites Facebook and Twitter to share their views on who they believed were to blame for the shooting.
Some believed the police were accountable and asked why live ammunition had been used. Others shifted blame to the company for not giving in to the demands of poorly-paid workers.
The Guardian reported that the shooting had led to a drop in the company's share price.
It said Lonmin's shares dropped almost 5% in London and 4% in Johannesburg.
Lonmin executives said all its shafts across the South African platinum belt, which account for 12% of global supply, were closed to all but essential services, such as ventilation.
Although the price of platinum had fallen steeply in the past six months, the spot price rose 2% on Tuesday in the wake of the violence, it said.
Lonmin executive vice president Barnard Mokwena was quoted as saying: "Until the place is safe we don't want to talk about production."
South Africa accounts for about three-quarters of the world's platinum, which is used to make catalytic converters in cars.
London-based Lonmin told the Guardian that 96% of its production comes from Marikana.
Amcu leader 'wanted to die' The president of Amcu wanted to go back and die with his dead colleagues when he saw them fall during police fire at Lonmin's Marikana mine, he said.
"I told them to leave … I pleaded, I pleaded," Mathunjwa told reporters in Johannesburg.
The workers refused. They said they would stay, even if they were killed.
Tears rolled down his cheeks as he recounted the events that led to the deaths of over 30 protesters in a clash with police on a hill near the mine on Thursday.
He said workers had earlier refused to leave, vowing to stay on the hill even if they were killed.
"We got in our cars and left … After a few minutes the phone rang [about the shooting]. "I wanted to turn back and go and die with my comrades," said Mathunjwa.
Using a handkerchief to wipe his eyes, he said Amcu leaders went to the hilltop, where the protesters had gathered, before the shooting without a police or security escort, or any supervision.
"I pleaded with them: 'The writing is on the wall, they are going to kill you'," he said.
Mathunjwa said President Jacob Zuma should order a probe into the shooting. "It is with great regret … and shock … that this resulted in a loss of lives," he said.
He said the killings could have been avoided and called on the nation to mourn those who died.
Amcu would assist with funeral arrangements where it could. – Sapa