SYCAMORE, Ill. - Over the years, evidence decays or passes through so many hands it's rendered useless. Murder weapons disappear, and witnesses' memories dim or are carried to the grave.

Those are the kinds of obstacles that have stymied prosecutors trying to crack decades-old cold cases in recent years - and that now await authorities trying to build a case against a former Washington state police officer in the 1957 slaying of a 7-year-old girl in this northern Illinois farming community.

Can the little girl's friend, now in her 60s, remember with certainty what the suspect looked like? Will an unused train ticket, discovered half a century later, undermine Jack Daniel McCullough's alibi that he was in Chicago having a military medical exam the day that Maria Ridulph disappeared?

Cold-case experts say they rarely see arrests in murders that date back so many years.

"This is a very, very cold case," conceded Clay Campbell, the chief prosecutor in Illinois' DeKalb County, who is seeking the extradition of McCullough from Seattle to stand trial for the little girl's slaying. "I am well aware of the precarious nature of prosecuting a case you cannot prove, but we are confident that Mr. McCullough killed Maria Ridulph."

Campbell said he has never overseen a cold case that went to trial and never heard of one anywhere near as old as Ridulph's. "This has continued to be an investigation that has had its fits and starts," he said.

Maria Ridulph's abduction in the winter of 1957 grabbed the nation's attention, and President Dwight Eisenhower and FBI Director J. Edgar Hoover asked to be kept up-to-date about the progress of the massive search for the girl. Her body wasn't found until spring, by which point it was so badly decomposed that the cause of her death couldn't be determined.

A friend of Maria's, Kathy Sigman, told authorities that a young man who called himself "Johnny" approached them while they were playing and offered to give them piggyback rides. She said she left to get mittens, and that when she returned, her friend and the man were gone.

McCullough, whose name at the time was John Tessier, lived nearby but had an alibi. He told authorities he was in Chicago that day getting a medical examination before enlisting in the Air Force. He later moved out of the area, served in the armed forces and ultimately worked as a police officer in Washington and then a security guard at a retirement home - where he was arrested July 1.

Investigators reopened the case three years ago, after McCullough's girlfriend from 1957 told them she found his unused train ticket from Rockford to Chicago from Dec. 3, 1957, the day Maria vanished. Sigman, who is now Kathy Chapman, identified McCullough recently through a photograph of Tessier from the 1950s, which she said she was never asked to do half a century earlier.

At the news conference Tuesday, Campbell and an Illinois State Police spokesman refused to detail evidence in the case. But based on an affidavit made public by Seattle police and a jailhouse interview McCullough gave The Associated Press, a trial would certainly hinge on 50-year-old memories and faded, or even missing, documents.

McCullough told the AP he didn't use the train ticket because his stepfather gave him a ride to Chicago. And he said his "iron-clad alibi" would be supported by military records of the medical exam - except that officials at the St. Louis records repository confirmed Tuesday to the AP that his file had been destroyed in a 1973 fire.

With more than 200,000 unsolved killings since 1960, according to the FBI, cold cases have become a bigger part of prosecutors' jobs nationwide. But each case presents questions about how or even whether to haul aging suspects into court. In many cases, DNA evidence or other modernized technology opens the door to renewed prosecutions, but it's unlikely DNA was recovered or kept in the Ridulph case.

"It's a haunting thing," said Jennifer Coleman, a Cook County prosecutor who won a conviction in the 2002 retrial of a man charged in the 1955 slayings of three Chicago boys.

Rochester, New York-based district attorney Sandra Doorley knows the challenges better than most, with several cold-case convictions to her name, as well as a recent defeat.

She looked on as jurors earlier this year acquitted Willie James Kimble, 78, of sexually assaulting and bludgeoning to death a blind homemaker, Annie Mae Cray, in 1972. Among the potentially vital evidence that had gone missing was the alleged murder weapon, a bloodied log.

In other cases, prosecutors have physical evidence but aren't able to use it in court because it passed through so many hands that no investigator would vouch for it.

The passage of time also has stymied prosecutors trying to try high-profile 1950s civil rights cases. Among them was an attempted prosecution in a case related to 14-year-old Emmett Till, who was abducted and killed in 1955 after he whistled at a white woman.

"So many people who had been involved or could have shed more light on what actually happened had died," said Joyce Chiles, district attorney in Mississippi's Leflore County in 2004 when he could not convince a grand jury to indict a woman in the case.

Chiles recalled that another problem was the apparent loss of an item that played no small role in the horrific story of what happened to Till - the cotton-gin fan fastened to his body by his killers to make sure it sank in a river.

In rare cases, though, the passage of time also can help prosecutors.

In the 1955 case of the three Chicago boys, witnesses refused to tell authorities for decades how Hansen told them he'd raped young boys and warned them if they said anything they would "end up" like the three boys whose naked bodies were found in a forest preserve ditch.

"After time and distance, they came forward because they knew he could not hurt them anymore," Coleman said.