The case of Louis Mickens Thomas, a life prisoner whose contested murder conviction was detailed recently in The Inquirer, exposes a flaw in Pennsylvania's criminal-justice system that's largely unknown to the public: Convicts are often unable to access evidence that could prove their innocence because the state has destroyed it or isn't required to provide it.

Pennsylvania is among a minority of states without regulations on the retention of physical evidence or public records. It is also one of the few with blanket prohibitions against disclosing documents related to criminal investigations.

With every question of innocence that could have been resolved had the evidence been available, it becomes clearer that better disclosure and preservation policies would help ensure that only the guilty are convicted.

Under current state law, it's up to prosecutors to decide what information to disclose to a defendant before trial. While the U.S. Constitution requires prosecutors to turn over anything exculpatory, Pennsylvania lets prosecutors determine what does or does not meet that definition.

To be sure, state information should not be given unedited to defendants or defense attorneys before trials. But when it concerns a forensic evaluation of physical evidence, as with Thomas' case, or an identification of another person as the perpetrator - as in the case of Kenneth Granger, whose homicide conviction was overturned last summer - it should be disclosed.

In many states, and under federal law, defendants are given access to government files after conviction. In Pennsylvania, that's not the case. So evidence that could prove that the wrong person is in prison can sit forever undiscovered.

Rarely do prosecutors deliberately withhold exculpatory evidence, but they have mistakenly done so in numerous cases. Of the 261 exonerations involving DNA evidence nationwide, half involved a failure to provide exculpatory evidence to the defense at trial.

The potential for abuse is easy to see: Without any requirement that the prosecution hand over material, there's no way to determine whether the government has met its constitutional obligations. And a prosecutor who neglects or decides not to provide an exculpatory piece of evidence need not fear detection, as it will never be disclosed.

After a defendant has been convicted in Pennsylvania, information is even more difficult to obtain. Under the law, a defendant may not engage in a fishing expedition for exculpatory evidence. But the defendant has no way of knowing what potentially exculpatory information exists.

In Thomas' case, as in countless others, the uncertainty is compounded because the commonwealth apparently destroyed the evidence that could prove his innocence (or confirm his guilt). In most other states (and under federal guidelines), this would not have happened.

Thirty-two states provide for automatic preservation of biological evidence in certain cases; 23 require it in cases of homicides and sex offenses for the duration of a defendant's incarceration. But Pennsylvania doesn't require the government to keep evidence for any amount of time.

Preserving evidence makes sense for everyone, particularly victims of crime. After Charlotte, N.C., revamped its evidence preservation procedures, police were able to solve 15 cold homicides and make 14 related arrests. Pennsylvania State Police have used preserved evidence to arrest more than 250 criminals.

While there is no certain way to prevent all wrongful convictions, Pennsylvania can assure fewer of them with two steps: broadening the open-records law to allow disclosure of limited information related to investigations, and establishing standards for preserving physical evidence. The Advisory Committee on the Causes of Wrongful Convictions, which has been meeting since 2006, should include those measures in its recommendations. They would diminish the nearly insurmountable obstacles to establishing innocence and help ensure that our system convicts the guilty and frees the innocent.

We must fully exploit the available information and science for everyone - victims, innocents, the guilty, and all others who put their now-shaken faith in the criminal-justice system.

Marissa Boyers Bluestine is legal director for the Pennsylvania Innocence Project at Temple University's Beasley School of Law. For more information, see www.innocenceprojectpa.org.