The guards acquitted last Friday were accused of setting up prison yard fights among rival prison gangs in Corcoran Prison's High Security Unit between 1989 and 1995, and then shooting at them when brawls broke out. This resulted in guards shooting 31 unarmed prisoners, seven of them fatally.

'The incidents at Corcoran were part of a serious pattern of abuses in the Californian prison system during the period in question,' Amnesty International said. 'We are worried that investigations into this and other cases were hampered by cover-ups and the ‘code of silence' among prison officers.'

Amnesty International is concerned by claims that the prosecution was stifled by the judge's ruling barring any testimony about a 'code of silence' among prison guards. A former guard called as a government witness backed away from her testimony for fear of being labelled a 'rat'. The two prison guards who became 'whistle blowers' and exposed the deadly system at Corcoran were subsequently threatened, ostracised and forced to resign.

The prosecution was further hampered by the judge allowing several people to sit on the jury even though they apparently had reason to lean toward the guards' side, including someone whose application to be a state guard was pending.

'The shootings during the period of the gladiator fights raise serious questions about the failure to ensure a safe environment for inmates and staff and about the use of lethal force on prisoners,' Amnesty International said. Between 1988 and 1994, more prisoners were shot by guards in California than in the rest of the country.

'The authorities should explain why there have been so many more shootings in California than in any other state,' the organisation added.

While the Californian authorities have been forced to revise their policy on the use of deadly force and 'integrated yards', Amnesty International continues to receive reports on the failure of prison officials to protect inmates.

'Adequate preventative policies and monitoring are urgently needed,' the organisation said. 'It is time the Californian authorities put into place measures to deal with the ‘code of silence' among prison guards to break the circle of connivance in abuses against inmates .'

Amnesty International is also urging the authorities to ensure that any prison officer found to have been involved in abuse is strongly disciplined or removed from the prison system. The organisation has recently written to the authorities about a series of cases of alleged abuse in Californian prisons.

Background

California's prison system is the largest in the country. State legislative hearings in 1998 confirmed a pattern of brutality at Corcoran, and criticised the Californian Department of Corrections for failing to investigate or prevent abuses in prisons and for inadequate discipline. An independent panel found that nearly 80 per cent of the shootings were unjustified and never fully investigated by the state.

The trial of the eight Corcoran guards was one of the largest prosecutions ever of state prison guards and eventually took place in the federal courts after the abuses and cover-ups were exposed by two 'whistleblowers' -- and after four years of Federal Bureau of Investigation (FBI) and newspaper investigations. The prosecution conceded, however, that it would be difficult to prosecute guards in a largely conservative area with a high number of prisons.

The eight guards were indicted in 1998 after years of internal investigations and legislative hearings had produced no charges. Four of them had faced possible life sentences for civil rights abuses including the fatal shooting of inmate Preston Tate during one of the 1994 brawls, moments after a guard allegedly said: 'It's going to be duck hunting season'.