SPRINGFIELD — Defrocked Catholic priest Richard R. Lavigne died last week as police were poised to seek an arrest warrant charging him in the 1972 slaying of altar boy Daniel “Danny” Croteau, Hampden District Attorney Anthony D. Gulluni said Monday.

Citing new evidence that included hours of recent interviews with Lavigne and a forensic linguist’s review of a letter seen as a key piece of evidence in the case, Gulluni said the investigation into Croteau’s death is now officially closed.

The district attorney gave the green light Friday to state police detectives to present the case to a magistrate.

“We were prepared to then prosecute Richard Lavigne and prove his guilt beyond a reasonable doubt,” Gulluni said.

But Lavigne, 80, died that day. Gulluni said the priest was “not well,” but did not provide a specific cause of death.

The developments came after Lavigne, in a series of interviews this year with a Massachusetts state trooper, admitted driving Croteau to a riverbank in Chicopee in April 1972, slapping him, shoving him and beating him with an “object” he then tossed into the water.

Lavigne said he left Croteau alive near the Chicopee River but drove back to the scene a short time later and saw the boy floating face down in the water. He told no one, he reported to the trooper.

“Lavigne said that he then started ‘bawling’ crying. Lavigne said ‘people are going to blame me, you know? And they did, it was the worst experience of my life,’” read a statement of facts the trooper prepared in order to obtain an arrest warrant.

“If I shared this with the public, they wouldn’t believe it,” Lavigne told the trooper in a recording Gulluni played during a press conference Monday.

But Lavigne still insisted in the interviews that he did not kill Croteau, whom witnesses said spent nearly every weekend on overnights with the priest. Lavigne was eventually exposed as a prolific child molester who was shuffled from parish to parish in Western Massachusetts during the 1960s until the early 1990s.

Lavigne consented to 11 hours of interviews while at a medical facility in Greenfield, the trooper’s statement said. Had he not died, his arrest would have been a stunning development in the 49-year-old unsolved case that featured Lavigne as an elusive prime suspect.

The case frustrated investigators for decades.

“We tried every DNA test you can think of,” former Hampden District Attorney William M. Bennett said in 2008. “Every time we go down a particular path we come up empty.”

Croteau’s body, his face and head battered by a blunt instrument, was found on April 15, 1972, under the Robinson Bridge in Chicopee Falls. An autopsy showed he died of fractures of the skull and lacerations of the brain. He was wearing his Catholic school uniform and had been drinking heavily before his death, tests revealed.

Witnesses told police Lavigne would often ply them with liquor and copies of Playboy magazine, which he kept stuffed under the seat of his car.

Lavigne was criminally charged with sexual abuse of children in the early 1990s and pleaded guilty to two counts of indecent assault and battery on a child. The abuse occurred during the 1980s. Lavigne escaped a jail sentence and instead served 10 years of probation. The diocese removed Lavigne from ministry after his arrest, but he was not formally defrocked for another decade.

The diocese paid $1.4 million to multiple victims of Lavigne’s sexual abuse — including Croteau’s brothers.

Before he died, Lavigne told Trooper Michael McNally he “slapped Croteau a little bit” to “shut him up, or get him to stop doing what he was doing.”

“Lavigne was asked ‘what was he doing?’ and he replied ‘frankly I don’t remember.’ Then Lavigne was asked ‘what did you do with the object after you hit him? He replied ‘tossed it in the water,” McNally’s narrative reads.

Troopers with the district attorney’s office re-interviewed witnesses who had been around Lavigne and Croteau, including someone who called himself Croteau’s “best friend” who reported the boy would burst into tears when Lavigne arrived in his car to pick the boy up after school. Another witness remembers Croteau yelling “I’ll tell! I’ll tell!” at Lavigne while a group of them were camping in Goshen.

It was shortly after the vague threat to expose Lavigne that Croteau was killed.

This story will be updated upon further reporting.

Related content:

• ‘Significant development’ in 1972 slaying of altar boy Danny Croteau to close case
• February 2008: Court documents reveal altar boy’s ordeal