MARBLEHEAD — A long-ago social connection between a Marblehead police officer and a relative of Thomas Larivee, 19, was the key reason for taking the investigation of Allie Castner's death out of the hands of the Marblehead police, according to Chief Robert Picariello.

"It's a possible, perceived conflict of interest," Picariello said yesterday. Worcester District Attorney Joe Early Jr.'s decision to remove local law enforcement from the case was taken "in an abundance of caution," Picariello said. He added there was no suggestion that any of his officers had acted to shield Larivee.

Picariello learned in late September that Sgt. Marion Keating had a potential conflict, having hosted a wedding for a Larivee relative 23 years ago. He immediately informed Early, who had recently taken control of the legal side of the case, putting it in the hands of Assistant District Attorney Blake Rubin.

This week, Early took the further step of assigning investigative duties exclusively to state troopers in the Worcester office. It was the latest turn in a case where officials declined to release much information only to see the matter spin out of control as the public and the Castner family reacted with suspicion and distrust.

Essex County District Attorney Jonathan Blodgett had earlier bowed out of the case — also because of the perception of a conflict of interest stemming from his own professional relationship with Larivee's uncle.

Initially, Early spokesman Tim Connolly said his office would still work with the investigators already on the case, Marblehead police and state troopers in Blodgett's office.

The 15-year-old Castner was killed on Aug. 24 when she was struck by Larivee's gray Volvo while crossing Pleasant Street near Mohawk Road. The fact that no charges have been leveled at Larivee contributed to growing mistrust from the Castner family.

A lack of witnesses might be to blame for the slow pace of the investigation, but so few details have been released that it's difficult to judge.

Last month, the Castners went public with their doubts. Grief-stricken dad Chris Castner told a meeting of the selectmen that he wanted Larivee charged. Other family members sent letters to media outlets raising questions about, among other things, the Salem man's checkered driving record.

Shortly after, the Castners met with Blodgett, and the district attorney made his decision to hand over the matter to Worcester only hours later.

"I'm not suggesting other than an appearance of a conflict of interest," said Gerry D'Ambrosio, a lawyer representing the Castner family, commenting on Keating's involvement and the move to bring in Worcester investigators. "We welcome the decision only because it's been our position that it's better to have fresh eyes look at the case."

D'Ambrosio declined to endorse the family's demand that Larivee be charged, saying instead, "They simply ask for a complete, impartial, thorough investigation."

Explaining that he's no civil attorney, D'Ambrosio describes his role as a kind of watchdog for the Castners, making certain that a potential criminal case is handled properly. A former assistant district attorney in Suffolk County, he said the family has raised several red flags on a number of issues regarding the investigation thus far.

He declined to go into specifics, but mentioned Blodgett and Picariello's refusal to release Larivee's name to the media — it was eventually confirmed by the Registry of Motor Vehicles. "My DA's office routinely disclosed the name," he said of similar cases.

"It was an ongoing investigation," Picariello said, and releasing the name was thus not appropriate.

The Castners were also alarmed by the treatment of witnesses, D'Ambrosio said. "They questioned why some people were questioned and some were not." Chris Castner has publicly pleaded for witnesses to come forward. "Why is Chris Castner going through names of witnesses when they (the police) should be coming up with those names?"

If any favoritism was shown by Marblehead or Essex County officials it could go unrecorded. In Worcester, Connolly at first resisted the idea that his office would even comment if such wrongdoing was discovered. "We've been asked to investigate the accident," he said, insisting that Early's mandate is narrowly defined.

After a reminder that the public trust could hinge on full disclosure, he conceded that such information would likely be turned over to the appropriate authorities.

A Blodgett spokesman declined to comment on "a pending case," citing ethical considerations.