PROVIDENCE — The state Supreme Court on Monday overturned the 2006 conviction of a former Rhode Island State Police trooper accused of assaulting a man in South Kingstown police custody as he sat in the back of a police cruiser.

The court found that the efforts to exclude evidence from Jeffrey L. Clark’s jury trial violated his constitutional right to cross-examination. The justices were unable to conclude that the error was harmless beyond a reasonable doubt, so they vacated the conviction and returned the case to Washington County Superior Court for a new trial.

The high court also reprimanded the attorney general’s office for filing broad in limine motions — which allow the court to exclude evidence that is inadmissible by law or could be prejudicial.

“Too often do these motions impact the constitutional safeguards guaranteed to criminal defendants under the state and federal constitutions, leaving this court with no alternative but to vacate these convictions,” the court wrote. “We therefore admonish the state to wield its in limine sword carefully…”

Before the trial began, the state was granted motions that prohibited Clark from questioning his alleged victim, William Skwirz Jr., about his alcohol consumption prior to the men’s altercation in Clark’s neighborhood, and from cross-examining witnesses about a civil settlement between Skwirz and the Town of South Kingstown.

“We agree with the court’s position that motions in limine should be used with specificity, but like all motions, attorneys make the motion, and it is up to the trial court to rule upon it,” said Beryl Kenyon, spokeswoman for the attorney general’s office.

In June 2006, a Washington County Superior Court jury convicted Clark, of 254 Laurel Lane in South Kingstown, of assaulting then 22-year-old Skwirz as he sat handcuffed in the back of a police cruiser on Labor Day weekend 2004.

Skwirz, of Narragansett, had been celebrating his stepbrother’s return from Iraq at his father’s house, at 274 Laurel Lane, early on Sept. 5 when Clark returned home from a wedding and began yelling about a barking dog. The two argued, and witnesses testified that Clark threw the first punch. Clark, a state trooper who was off-duty at the time, said he acted in self-defense.

Witnesses testified that after police officers arrived and arrested Skwirz, Clark repeatedly punched him as he sat handcuffed in the back of the cruiser. Clark denied the charges, as well as the prosecution’s claim that Clark tried to enlist South Kingstown officers to cover up the incident.

During the trial, the judge forbade Clark’s lawyer to question Skwirz about a financial settlement he had received after he approached the town about liability for his injuries. The defense also was not allowed to disclose that Skwirz spoke to state police investigators while his liability claim was still pending. The restriction “deprived defendant of an opportunity not only to attack Skwirz’s credibility, but to show that he had a motive to lie for a financial gain,” the court ruled.

In January 2007, the state police rescinded Clark’s right to reenlist, citing nine administrative charges related to the Skwirz incident, said state police Maj. Elwood M. Johnson. A three-member internal panel upheld the decision after a just-cause hearing held at Clark’s request.

Attorney General Patrick C. Lynch said he was “disappointed” that a new trial had been granted, though he seemed pleased that other questions about the case had been clarified in preparation for the new trial.

“As attorney general, I hold our state’s police officers in very high regard, but when allegations of wrongdoing by a police officer — even if off duty as in this case — are raised, those allegations must be taken seriously.”

tbuford@projo.com