After hearing more than seven hours of testimony from investigators and witnesses, a Leflore County grand jury last week determined there was insufficient evidence to indict Carolyn Bryant Donham on charges of kidnapping and manslaughter, Leflore County District Attorney Dewayne Richardson said.

The decision comes despite recent revelations about an unserved arrest warrant and the 87-year-old Donham's unpublished memoir.

The Reverend Wheeler Parker Jr, Emmett Till's cousin and the last living witness to Till's August 28, 1955, abduction, said Tuesday's announcement was "unfortunate, but predictable".

"The prosecutor tried his best, and we appreciate his efforts, but he alone cannot undo hundreds of years of anti-black systems that guaranteed those who killed Emmett Till would go unpunished, to this day," Parker said.

In June, a group searching the basement of the Leflore County Courthouse discovered the unserved arrest warrant charging Donham, then-husband Roy Bryant and brother-in-law JW Milam in Till's abduction in 1955.

While the men were arrested and acquitted on murder charges in Till's subsequent slaying, Donham was never taken into custody.

The 14-year-old Chicago boy was visiting relatives in Mississippi when he and some other children went to the store in the town of Money where Carolyn Bryant worked.

Relatives told the AP that Till had whistled at the white woman, but denied that he touched her as she had claimed.

In an unpublished memoir, Donham said Milam and her husband brought Till to her in the middle of the night for identification but that she tried to help the youth by denying it was him.

She claimed Till then volunteered that he was the one they were looking for.

Till's battered, disfigured body was found days later in a river, where it was weighted down with a heavy metal fan.

The decision by his mother, Mamie Till Mobley, to open Till's casket for his funeral in Chicago demonstrated the horror of what had happened and added fuel to the civil rights movement.

Following their acquittal, Bryant and Milam admitted the abduction and killing in an interview with Look magazine. They were not charged with a federal crime, and both have long since died.

In 2004, the US Justice Department opened an investigation of Till's killing. It later said the statute of limitations had run out on any potential federal crime, but the FBI worked with state investigators to determine if state charges could be brought.

In 2007, a Mississippi grand jury declined to indict anyone, and the Justice Department closed the case.

Federal officials announced last year they were again closing their investigation, saying there was "insufficient evidence to prove beyond a reasonable doubt that she lied to the FBI".

Australian Associated Press