Johannesburg (CNN) -- A British private security firm is denying claims its employees administered electric shocks and injected inmates with anti-psychotic drugs at a maximum security prison it ran in South Africa.
The claims came to light through Ruth Hopkins, a human rights investigator with the Wits University Justice Project, who said she started her probe after receiving dozens of letters detailing abuse at the Mangaung Correctional Centre near Bloemfontein, in the central part of the country.
Leaked videos also made their way to Hopkins, including one in which she says the sound of electric shocks can be heard, and someone screaming in excruciating pain.
The security company, G4S, told CNN it does not use any form of torture or shock treatment, and that G4S staff and Mangaung Correctional services do not administer medication nor have access to it.
G4S insists only independent, certified medical staff administer medications, not G4S security personnel.
"The role of G4S staff is to prevent prisoners who are being treated from causing harm to other prisoners and staff. This is in accordance with the relevant legal guidelines," the company said in a statement on its website Monday.
A representative for the company said the allegations are being taken very seriously and the company would be launching its own investigation into the matter. The South African government is also investigating the actions of G4S and the independent medical team based at the prison. The results of that probe could come as early as Friday.
Although it no longer runs the facility, G4S said in its statement there is "an active and independent inspection regime in place at the Mangaung prison, and there have never been any allegations or concerns regarding anything of this nature."
Hopkins told CNN that her investigation also revealed "that the prison was also forcibly injecting these inmates with anti-psychotic medication, according to the accounts of the inmates."
One video leaked to Hopkins shows uniformed G4S employees holding down a struggling inmate as he is given an injection.
"I am not an animal!" he is heard screaming.
Hopkins said a health care worker reported that a certified medical staffer who is not a G4S employee gave the inmate an anti-psychotic drug despite no record of psychosis.
The videos were shot by G4S employees themselves. According to its contract with the South African government, the security team is obligated to document its actions and only allowed to use force under strict guidelines.
The company says it cannot verify the authenticity of the videos and strongly denies all allegations of mistreatment at the prison, which houses 3,000 inmates and is the second-largest private prison in the world, according to G4S.
Documents obtained by Hopkins suggest the problems at the prison have been going on for some time.
In 2010, a Department of Correctional Services employee filed a classified memo to his superiors stating that the "state is being milked for work not done" and that the company uses "the cheapest methods" at the prison, describing the use of electric shock as routine.
More than 300 G4S employees at the prison went on strike earlier this month, complaining of being ill-equipped and understaffed. The company responded by firing hundreds of them, which was followed by more violence at the notorious prison.
The government ended up firing the firm, saying it had "lost effective control over the facility."
GS4 has contracts across the globe, including with U.S. Customs, Baghdad International Airport security, the Wimbledon tennis championships and dozens of red-carpet events.
But it's not the first time the company has come under scrutiny. GS4 won the lucrative London Olympics contract to provide security staff for events, but wasn't able to perform all its duties, forcing the British government to call in additional troops to secure the games.