BOSTON (AP) _ An alleged gang leader who faced prosecution by an assistant attorney general was indicted Thursday for the man's execution-style slaying.

Jeffrey Bly, 23, was indicted for first-degree murder by a Suffolk County grand jury in the killing of Paul McLaughlin.

Bly, who is serving a prison sentence for carjacking stemming from another case, is accused of shooting McLaughlin once in the head after the prosecutor got into his car at a Boston commuter rail station on Sept. 25, 1995.

McLaughlin, 42, worked as an anti-gang prosecutor on assignment to the Suffolk County District Attorney's Office. The former candidate for state representative was killed the night before he was scheduled to try Bly on the carjacking charge.

The killing was the subject of one of the most intensive investigations in recent Boston history.

``It's both a relief and a real emotional day,'' Tom Green, an assistant attorney general and a close friend of McLaughlin, said as the indictment was announced.

The document itself was sealed. It was scheduled to be released Friday, when Bly was to be arraigned in Suffolk Superior Court. McLaughlin's family, Attorney General Scott Harshbarger and others planned to attend the arraignment.

``The prosecution of Jeffrey Bly will not fill the void or ease the pain,'' McLaughlin's father, former Lt. Gov. Edward F. McLaughlin Jr., said in a statement. ``We are, however, only interested in justice and seeing that Paul's murderer is convicted and punished in accordance with the law.''

Thomas Brennan, a special assistant attorney general assigned to handle the case, said Thursday he could not comment on details of the investigation.

At the time of the killing, attention immediately turned to McLaughlin's anti-gang work. Investigators reviewed a list of alleged gang members prosecuted by McLaughlin, and questioned many of them, including Bly, who was known on the streets as ``Black'' because of his penchant for black clothing.

McLaughlin had tried Bly twice before on other cases. After McLaughlin's death, Bly was ultimately convicted on the carjacking charge, leading to the 10- to 15-year prison term he now is serving.

Bly's attorney, Joan Stanley, refused to comment Thursday.

Since the killing, a team of investigators from Boston and state police have combed through Bly's background. They also questioned his friends and allies on the city's streets, where Bly allegedly led a violent drug ring.