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 can not 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 was n't able to perform all its duties , forcing the British government to call in additional troops to secure the games .
The Mangaung Correctional Centre houses 3,000 violent offenders . The South African government fired the British security firm that ran the prison . Officials said G4S had `` lost effective control '' of the facility . Inmates , employees accuse firm of using electric shocks , other abuse .