Attorneys in South Carolina have said they have fresh evidence that warrants a new trial in the case of a 14-year-old black teenager executed nearly 70 years ago for the murders of two white girls.

George Stinney Jr was the youngest person to be executed in the United States in the 20th century.

Attorneys say the request for another trial so long after a defendant's death is the first of its kind in the state.

No official record of the original court proceedings exists; no trial participants are alive, and no evidence was preserved.

The law is unclear on whether any statute of limitations would prevent the case from being reopened.

Despite those obstacles, attorneys for Mr Stinney's family argued at a hearing that the crime that rocked the small mill town of Alcolu in 1944 deserves another look.

They filed a motion requesting a new trial in October based on newly discovered evidence.

Since then, new witnesses who could help exonerate Stinney have come forward, including a former cell mate who says the teen told him police forced his confession, attorneys said.

The defence is also relying on old newspaper accounts and a few records in state and county archives to make their case to a judge in Sumter, about 25km from the town where Stinney was tried and convicted.

Lawyers said they had determined Stinney was convicted solely on testimony by police who said the teen confessed to killing Betty June Binnicker, 11, and Mary Emma Thames, 7.

The two girls disappeared on 23 March 1944, after leaving home on their bicycles to look for wildflowers.

The girls rode a distance of about a mile to a railroad track that divided the segregated town, according to the defence's account of the case in court records.

Stinney and his younger sister Amie were sitting on the tracks as their family cow grazed nearby.

Stinney's sister recalls the girls asking where they could find flowers before both pairs of children went their separate ways.

Binnicker and Thames never returned home. A search party found their bodies the next morning in a shallow ditch behind a church.

Their skulls had been crushed and the bicycles laid on top of them.

After Stinney told someone he had seen the girls along the railroad tracks, he was picked up by police and held for five days before being arrested, said Matthew Burgess, one of the attorneys seeking a new trial.

The teen's family was run out of town, and his siblings never saw him again.