For the first time, Pennsylvania's law enforcement officers will have statewide guidelines aimed at preventing false identifications by witnesses, a persistent problem in criminal investigations.

"The point of all of this is very simple. That is to convict only the guilty, protect the innocent and stand up for the rights of the victims," said Tom Hogan, Chester County's district attorney.

Hogan, who is chairman of the Pennsylvania District Attorneys Association Best Practices Committee, which was formed in 2014 and developed the guidelines, announced the protocols at a news conference Tuesday with state law enforcement officials at the Chester County Justice Center in West Chester.

The guidelines recommend that officers present witnesses with photo arrays, showing multiple photographs of potential suspects, instead of one by one. To guard against subconscious bias, they recommend that an officer not involved in the case should administer the photographic line-up. The protocols also call for investigators to keep careful records of the suspect identification process.

The guidelines represent an attempt to keep up with evolving science and to identify what works best for witnesses. They also purport to standardize witness procedures across the state's 1,117 law enforcement agencies.

Eyewitness misidentification is the biggest factor in convictions of people later exonerated by DNA testing, according to the Innocence Project, a national nonprofit organization dedicated to clearing the wrongly convicted.

The guidelines are recommedations, and their usage will be up to each county's district attorney.

Decades of social science research has shown eyewitness accounts often are unreliable due to factors such as witnesses' stress levels during an incident, racial disparities, a lack of distinctive markings on a suspect, and the amount of lighting at a crime scene.

A 2014 report by the National Research Council recommended that agencies train law enforcement officers about the science of memory and practices to minimize the warping of memories. Officers should ask open-ended questions; minimize interactions among witnesses; document witnesses' level of confidence in their identifications, and limit chances to subconsciously influence witnesses, especially during police line-ups.

The Best Practices Committee received consulted local police departments, the Pennsylvania State Police, chiefs of police associations, criminal defense organizations, academic institutions, and officials in other states.

In coming months the committee will be looking at the use body cameras, officer-involved shootings, and the videotaping of interrogations.

610-313-8207 @MichaelleBond