ANGOLA, Louisiana -- Lawyers for Ryan Matthews, a 23-year-old black facing execution in Louisiana for a 1997 murder, have filed new DNA evidence which they say proves another man was responsible for the killing.

Matthews' case has attracted intense international criticism from human rights organizations, such as Amnesty International. He was only 17 when arrested for the shooting of grocer Tommy Vanhoose and had a history of mental illness, AFP reported.

Amnesty members in Britain have sent thousands of cards to Matthews and U.S. prosecutors, and the organization is helping to fund a documentary film about the shooting.

His case has also been taken up by lawyer Barry Scheck, founder of the New York-based innocence project.

According to its website, the project "only handles cases where post-conviction DNA testing of evidence can yield conclusive proof of innocence." Scheck has helped free 127 convicts since the non-profit legal clinic was founded in 1992.

Billy Sothern, who represents Matthews along with the British anti-death penalty lawyer Clive Stafford Smith, said the original conviction relied too heavily on eyewitness accounts in the absence of forensic evidence tying Matthews to the crime.

Last year, Sothern said he heard rumors another convicted murderer, Rondell Love, was boasting he had killed Vanhoose. Love is serving 20 years in the state penitentiary for an unrelated murder committed eight months after the Vanhoose killing.

Love's DNA report was on file in his previous trial record, and Sothern discovered that it matched DNA material found in a ski mask worn by Vanhoose's killer.

The motion submitted by Matthews' lawyers also alleges that prosecutors withheld evidence from public defenders at the original trial, such as the fact that the main witness had pending felony charges against her.

"Prosecutors knew or should have known that there was DNA in the mask that would have cleared Ryan," Sothern said. "They knew or should have known that the main witness had pending felony charges. Whether this is a question of misconduct or incompetence, the result was the same for Ryan: He was sent to die."

Sothern also castigated the refusal of prosecutors to hear a review of evidence of Matthews' borderline mental retardation, which was submitted to them in August of last year.

Matthews' mother, Pauline, vigorously proclaims her son's innocence and insists that race was a key factor in his conviction.

"Eleven of the 12 jurors were white. Everybody in the court room was white except me and my family.

"They were in such a hurry to convict him. The police were joking with the victim's family -- it was like a big party," she said. Sothern also had harsh words for the long time the legal process has taken, saying "Every day that Ryan sits on death row is a travesty and an injustice."

He warned that Matthews has such inadequate medical care in prison given his mental condition, that he simply may not have enough time for the appeals system to work.

"Ryan has had abominable treatment on death row.

"He suffers from a serious seizure disorder. The medical staff at Angola have failed to give him his medication numerous times. Every time he has a seizure, he suffers irreparable brain damage," he said. However Pauline Matthews says her son remains hopeful -- "He's always believed he's going to be free."