Prosecutors in Colorado have cleared the parents of JonBenet Ramsey, a child beauty queen shown in this undated photo, of involvement in her unsolved homicide in 1996. (Denver Post/Associated Press)

The parents of American child beauty queen JonBenet Ramsey have been cleared of involvement in her unsolved 1996 killing by new DNA tests, prosecutors in Colorado said Wednesday.

The tests point to an "unexplained third party," Boulder County district attorney Mary Lacy said.

Prosecutors don't consider any member of the Ramsey family to be a suspect, Lacy said as she released a copy of a letter to the Ramseys that also contained an apology for the failure to find the killer.

"We intend in the future to treat you as the victims of this crime, with the sympathy due you because of the horrific loss you suffered," the letter says.

The six-year-old Ramsey was found strangled and bludgeoned to death in the basement of her Boulder, Colo., home late in the day on Dec. 26, 1996.

Her mother, Patsy Ramsey, had found a ransom note early that morning demanding $118,000 US for the safe return of the child.

A frantic search for the girl ensued, her parents have said, lasting until later in the day, when JonBenet's father, John Ramsey, found the child's body hidden in the cellar of their mansion.

Her death sparked feverish media coverage and pressure on the authorities to resolve the case, but investigators consistently failed to make any breakthroughs.

The investigation into her death was plagued by forensic errors and disputes between police and prosecutors. A year-long grand jury investigation ended in 1999 with no indictments.

The Ramseys came under intense suspicion in connection with the crime and later moved out of state. The letter from prosecutor Lacy to the Ramsey family also apologizes for the role authorities played in the public's mistrust of the girl's parents.

"To the extent that we may have contributed in any way to the public perception that you might have been involved in this crime, I am deeply sorry," Lacy says in the letter.

A man who was alleged to have confessed to the killings in e-mails to a Colorado journalism professor was arrested in Thailand in 2006 and extradited to the United States, but he was later cleared by investigators of any involvement in the crime.

Patsy Ramsey died in June 2006 at the age of 49 after a long battle with ovarian cancer.

John Ramsey, a millionaire businessman, unsuccessfully ran for a seat in Michigan's House of Representatives in 2004.

He has said many times in interview that neither he nor his wife had anything to do with their daughter's death and that he was confident her killer would be caught.

With files from the Associated Press