Since it reintroduced the death penalty a quarter of a century ago, America has executed 695 people. Another 3,703 wait on death row. But a recent spate of releases of the wrongfully convicted has triggered alarm that America may be executing the innocent.

Stacey Delo, former journalism student whose investigation freed the wrongly accused

Stacey Delo and Laura Sullivan are former journalism students of America's Northwestern University in Chicago, Illinois.

They were given a possible miscarriage of justice to investigate: the case of four young black men convicted in 1978 of a double murder and rape.

The victims were a young white couple, Larry Lionberg and Carol Schmal. They were abducted from a petrol station in a mainly black suburb of Chicago, and taken to an abandoned house, where their bodies were found the next day.

Since the death penalty was restored more than 90 death row inmates have been proved to be innocent and freed.

Charles Wheeler

Within hours police arrested the four and announced that they had solved the crime. At trial the prosecution produced a star witness, a teenaged girl named Paula Gray, who claimed to have watched the accused men raping Carol Schmal before killing her.

The jury accepted her story. Two of the accused were sentenced to death, a third was given life without parole and the fourth was sent down for 70 years.

Laura Sullivan with wrongly convicted Kenny Adams

Stacey and Laura spent months looking through documents. They visited two of the convicted men and found a suppressed police report pointing to different suspects. Then they called on Paula Gray, who told them she had made up her testimony after being threatened by the police.

Finally they two students persuaded the men named in the police report they had discovered to confess. DNA tests clinched the case. The four wrongly convicted men were exonerated and released. But they had lost 18 years of their lives.

US justice system questioned

In 1973 the US Supreme Court struck down the death penalty on the ground that it was applied capriciously and unfairly, and therefore violated the constitutional ban on cruel and unusual punishment.

Three years later a more conservative court restored it. Since then more than ninety death row inmates have been proved to be innocent and freed - enough to make a growing number of Americans wonder aloud whether the system of capital punishment can be trusted to deliver justice.

Governor George Ryan, Republican, Illinois

Liberals who were pressing for abolition for years have now been joined by supporters of capital punishment. Last summer the Republican Governor of Illinois, George Ryan, stopped all executions in his state. The number of exonerated inmates exceeded the number actually put to death.

Executing the innocent

In Virginia, a state with a rate of executions that is second only to Texas, a member of the state parliament has just tabled a bill to take the death penalty off the statute books.

Twenty years ago Frank Hargrove sponsored a bill to introduce public hanging from a gallows to be set up in front of the state's Capitol building in Richmond.

Seven out of 10 death sentences were reversed because of serious error in the original trials.

Charles Wheeler

Three things have prompted his U-turn: the murder rate in the 38 states that execute people is no lower than that of 12 states that have no capital punishment; life imprisonment without parole keeps murders off the streets while avoiding the finality of execution; and his fear that the state may have executed the innocent.

Chiefly its these narrow escapes from the electric chair and lethal injection that have fed America's debate about the death penalty.

Damning research

What may in the end prove even more telling, however, is the result of a study by the Columbia School of Law in New York.

After examining every capital punishment case passing through the appeal courts between 1973 and 1995 its lawyers found that seven out of 10 death sentences were reversed because of serious error in the original trials.

The electric chair - symbol of capital punishment

To its authors the study suggests that America's capital punishment system is breaking down under the weight of its own mistakes. They point out that an appeals process burdened with the task of catching so much error is hugely expensive. Also it takes an inordinate amount of time.

Clearly nearly 70% of America's 3,700 death row inmates should never have been sent there. And although not more than about one in 10 will ever be executed, the wrongly sentenced are having to wait for anything from 10 to 18 years to be told of their fate. That's not unusual, and it is certainly cruel.

Death Row on Trial: 1830 GMT, Saturday 17th February on BBC 2.

Reporter: Charles Wheeler

Producer: David Taylor

Deputy Editor: Farah Durrani

Editor: Fiona Murch