DALLAS (Reuters) - James Waller spent 10 years behind bars and waited another 14 years -- when his movements were restricted as a sex offender -- before DNA evidence finally cleared his name.
He is one of the lucky ones, according to the Innocence Project, a New York-based organization dedicated to exonerating wrongly convicted people.
DNA testing allows crime laboratories to compare genetic evidence at a crime scene, such as semen, with the DNA of a suspect.
Some 218 people have been exonerated in the United States using DNA since the technique was first used to overturn convictions in 1989.
But for many others, the evidence has either been lost or simply was not preserved, the Innocence Project says.
Only 25 of America's 50 states and Washington have legislation compelling authorities to preserve evidence of old cases. Even where such laws exist, they are often inadequate, while storage procedures and facilities are poor.
"In New York City we have around 20 cases we are working on where the evidence simply cannot be located. It's unavailable to us for testing and we don't know if it is lost or if it has been destroyed," said Rebecca Brown, a policy analyst at the Innocence Project.
Waller, 52, was convicted in 1983 of sexually assaulting an adolescent boy. He spent 10 years in prison and for 14 years he was registered as a sex offender, preventing him from visiting his nieces or going where children might be present, such as parks or basketball games.
"A dog could go where I couldn't go," Waller said in an interview in an east Dallas coffee shop.
The jurisdiction where Waller was convicted, Dallas County, Texas, leads the country in such cases, with 17 people cleared by DNA evidence. An 18th person was scheduled for release on July 3.
An aggressive district attorney goes a long way to explaining the number of wrongful convictions, according to Fred Moss of the Dedman School of Law at Southern Methodist University in Dallas.
"A lot of these cases occurred under the Wade administration and the culture of that office was aggressive, advancement was based on conviction rates," Moss said.
He was referring to the late Henry Wade, the Dallas District Attorney for more than 35 years, who oversaw famous cases such as the prosecution of Jack Ruby for the murder of Lee Harvey Oswald, the assassin of President John F. Kennedy.
The policy of the District Attorney's office during the 1980s, when many of these trials occurred, was to "nail scalps to the wall," Moss said.
Another factor is racism in a big town in the U.S. South. Of the 17 people whose convictions were overturned on DNA evidence in Dallas County, 13 of them are black. Nationally, two thirds of the 218 exonerated were black.
In Waller's trial, the victim, who is white, described an attacker much smaller than Waller, who is six feet four inches tall. Asked to identify his attacker in court, he pointed to Waller's lawyer, another African American male, who was about five feet eight inches.
Ironically, one of the reasons that Dallas has turned up so many wrongful convictions is that it preserves physical evidence under a policy aimed at reconvicting anyone whose conviction is overturned by an appeals court.
In other states, preserved evidence is harder for lawyers to get hold of.
In Georgia, evidence in death penalty cases is kept until the convict is executed, but for serious felonies that do not carry the death sentence, such as rape, storage is only mandatory for up to 10 years after judgment.
Louisiana and Virginia only require preservation in death penalty cases upon conviction.
"Dallas is the canary in the coal mine. It is indicative of a larger problem that is not unique to Dallas," said Moss.
With so many of these cases emerging and many more expected to come to light, analysts say it stands to reason that an innocent person may have been among the more than 1,100 convicted killers executed in the United States over the past three decades. More than 400 of those executions have occurred in Texas.
This view is echoed by Dallas District Attorney Craig Watkins, who took office 18 months ago.
"There probably has been someone not just from Dallas County but from this state that was executed for a crime they didn't commit. Reason would tell us that," he told Reuters.
Watkins, Texas' first black DA, also said he was mindful of charges of racial bias in the dispensation of justice in the past and said his office was determined to investigate all possible cases of wrongful conviction. "We get letters every day from individuals claiming innocence," he said.
But does the use of DNA and other improvements in forensic investigations mean that the number of wrongful convictions is in decline?
The Innocence Project says tens of thousands of prime suspects have been shown by DNA testing to have been wrongly accused.
But it also says its caseload is growing and that flawed forensic testing, eyewitness misidentification, false confessions -- and the fact that DNA is not always available -- mean that the wrong people still go to jail.
For James Waller the DNA evidence came too late.
He talks about how he used to come with his wife to the same coffee shop where he now sits. She was killed in a car accident in 2001 when she was eight months pregnant.
Choking back emotion, he said: "They took away a whole generation from me. I wanted to have kids but never had any."
(For a FACTBOX on exonerations, click on)
Reporting by Ed Stoddard; Editing by Bruce Nichols and Eddie Evans