LOWELL, Mass. >> Even though he has answers now after so many years when there were none — his grief scabbing over but never healing, his stubbornness and persistence all that kept him going — William McCabe is still adding to the book about his son.

He pores over its pages, editing here, scribbling there, his hands, at 83, not as dexterous as they used to be.

“Don’t look at the back,” he says. “There’s stuff there that I’m going to give the police.”

He wrote the first words 41 years ago, days after the body was found in a rain-soaked vacant lot on Maple Street here, his 15-year-old son, Johnny, strangled, tape over his eyes and mouth, a thick white rope around his neck, his wrists and ankles bound.

“It must take a special form of madman to senselessly murder a child, the effect of which could drive a family insane,” McCabe wrote after the murder.

In the book, he recalled Johnny’s baby steps, his early utterings (“Gah gee gay!” he would scream when he heard church bells ringing, mistaking them for the ice cream truck), how he would shake hands with his mother and father and say “God loves you” before he went to bed.

He recorded the ice-fishing trips, the baseball games, Johnny’s passion for fixing lawn mowers and his love of animals. And over the next four decades, he meticulously documented every clue he could find, no matter how tiny — a friend who saw Johnny at the Knights of Columbus dance in Tewksbury that Friday night; another who knew someone who knew someone who might have heard something.

He passed what he learned on to the police, calling them early in the morning or late at night, asking, “Anything new with the McCabe case?,” reminding them about his son, reminding them not to let Johnny be forgotten.

“Perhaps as the killer gets older and watches his children or his friends’ children grow, and as he reaches out to touch them, he will think of our son and our loneliness and sadness,” McCabe wrote.

Then, last month, the news came: three men, arrested in connection with the murder of John Joseph McCabe on Sept. 26, 1969.

The men were teenagers themselves at the time — tough boys, some of Johnny’s friends have told the McCabes — but much older now, their faces weary after the passage of four decades, after years of work, marriages and divorces, stints in the military. Weary also, perhaps, from the strain of hiding a terrible secret for so long.

All three, the McCabes realized, had attended Johnny’s wake and signed the guest book laid out for mourners.

Walter Shelley, now 60, worked at a box-making plant at the time of his arrest and lived with his wife in the same green, two-story frame house on Nelson Street in Tewksbury where he had grown up, a couple of miles from the McCabes’ house. Michael Ferreira, 57, was a forklift driver on medical leave from the Coca-Cola plant in Lowell and was often seen by his neighbors in Salem, N.H., walking with his wife and their dogs to a nearby pond, according to local news reports.

Allan E. Brown, 59, a retired Air Force reservist, seemed to have been “the weak link” in the chain, the police said. According to the police investigative report, he told them that Ferreira had threatened to kill him if he revealed to anyone what happened that night.

The Lowell policeB had long had suspicions about Shelley and Ferreira but did not have enough evidence to arrest them. Then, in 2009, Ferreira mentioned a new name, Brown’s, saying he had been with him that night, according to the police.

Detective Gerry Wayne, whoB had been obsessed with theB murderB for almost a decade,B was set to pursue the lead but he fell ill and died of cancer in 2009.B A new detective, Linda Coughlin, was assigned to the case this January. InB February, she and other investigators began a series of interviews with Brown, who at first insisted that he had nothing to do with the murder.

But at some point, Coughlin said, “I think he knew that we knew that he was lying.”

Eventually, the police report said, he told them how he and two other teenagers had driven around that night in Shelley’s 1965 maroon Chevrolet Impala, drinking and planning what they would do to Johnny McCabe.

They found him hitchhiking his way home and dragged him into the car, “punched and intimidated” him and then pulled him out, pinning him to the ground, Brown sitting on his legs, he told the police. They tied him up and left him on the ground struggling. A few hours later when they returned, Brown told the police, Johnny was dead.

The motive? Johnny, Brown told the police, had flirted with Shelley’s girlfriend, and they wanted to “teach him a lesson.”

To the McCabes, it seemed impossible that something so mundane could cost the life of their child. “When you change his diapers and bring him through chickenpox and measles, you expect to have that baby grow up into a man,” said Evelyn McCabe, Johnny’s mother.

In the years after Johnny’s death, killings over running shoes, sports team jackets, errant words of disrespect would become commonplace. But Tewksbury, in those days, was still balancing on the edge of innocence, a time when boys “sipped beer” and people left their doors unlocked.

For a few years after the murder, the police had hopes of solving it. They interviewed dozens of teenagers and adults. They administered polygraphs. A fingerprint pulled off the tape seemed promising, but it proved unidentifiable.

Rumors flourished: Johnny was murdered by a motorcycle gang, by a serial killer. Drugs were involved. The killers were imitating “Easy Rider” or some other popular film of the time.

But gradually, the leads petered out.

Inside the McCabes’ house in Tewksbury, Johnny’s room stayed just the way he had left it. For months, his mother set a place for him at the dinner table every night.

In the weeks after the murder, the McCabes and their two daughters all slept in the same bed. “We were all petrified,” William McCabe recalled. The couple kept their daughters close, interrogating them whenever they did manage to leave the house: “Where are you going?” “Who with?” “When are you coming back?”

Each morning, Evelyn McCabe would go to Mass and then visit Johnny’s grave, in winter brushing snow from the headstone. Roberta, 6 years old when her brother was killed, would sometimes fake illness to get out of school and go along.

“I didn’t want to leave her side because she was always upset,” she recalled.

Debbie, then 18, had trouble concentrating in school and soon left home to get married.

”It was really, really hard,” she said. “There were so many unanswered questions, and there were no answers and nobody to go to.”

“My mother would go downstairs,” she said. “She would sit on his bed. She would fold his clothes. For the longest time, she just kept everything.”

William McCabe could never really settle down, shifting jobs two or three times, up and down at night, writing notes that he put in the family Bible. Sometimes he and his wife would start to blame each other: She had not wanted Johnny to go to the dance that night; McCabe, just returned from a business trip, had overruled her. But in the end, the years of grief strengthened their bond, their daughters said.

After his son’s death, McCabe called the Police Department so often that everyone recognized his voice. “Some of them must have thought I was cuckoo,” he says. Yet it was his determination — his insistence on being “very appropriately pushy,” said District Attorney Gerard T. Leone Jr. of Middlesex County — that revived the investigation and kept it alive.

Capt. Jonathan C. Webb, head of the criminal investigations unit, said, “I really think that if Mr. McCabe hadn’t been so dedicated in always reminding us about the case, it would probably be a couple of boxes in a back room.”

A week or so before Easter — perhaps fittingly for a family who had never lost their strong faith — Coughlin called the McCabes and told them there had been developments in the case.

“It was a proud moment, just to be able to say to this man who had pushed so hard for so long, ‘We finally have some answers,’ ” she said.

Now what secrets remain are likely to come out in court. Shelley and Ferreira were charged with murder, Ferreira in juvenile court because he was 16 at the time of the crime. Ferreira was also charged with perjury; he lied to a grand jury looking into the murder, the police said. Brown was charged with manslaughter. All three have pleaded not guilty. A pretrial conference is scheduled for May 26.

On April 30, Brown was arrested in Londonderry, N.H., where he lives, and was accused of threatening to shoot his wife and himself with a .38-caliber pistol, an affidavit filed in the case said. Brown’s wife, Carolyn, told the police that her husband had had violent outbursts in the past and told her his impending trial would “only end in death.”

At first, for the McCabes, it was as if it was all happening again. But in recent weeks, they have felt a kind of relief. People have called and sent flowers. Friends of Johnny’s have visited, hugging the McCabes and reminiscing.

Yet when a child is murdered, there is perhaps no such thing as closure. And so William McCabe keeps writing in his book, adding accounts of the visits and any new details that emerge.

“Perhaps the murderer will read this and say, ‘I thought it was just another punk kid that I strangled to death,’ ” he wrote long ago in the weeks after his son died. “It wasn’t. He was very special to us, so we have written this book about him.”