“I felt trapped,’’ McGrath would later tell a psychiatrist, who diagnosed him as a paranoid schizophrenic. “The night it happened I had the idea that with a small movement of the trigger I could end it.

But underneath the outgoing exterior was a scared teenager growing increasingly depressed at the thought of failing to live up to expectations of him. College, work, family, and love were all looming pressures he felt could be relieved only through death.

McGrath - who, if alive, is 67 years old - was 17 when he murdered his family at their home in this industrial town on the banks of the Sugar River. The white house with the stone steps still sits on Pike Hill Road. McGrath, voted “class intellectual’’ at Towle High School, had applied to Dartmouth College and the University of New Hampshire and was preparing to play the lead in the class play, “The Male Animal,’’ according to newspaper reports at the time.

Since he walked off the hospital grounds, there has not been a single, solid lead about the location of McGrath, who has remained on the run longer than any other escapee in New Hampshire history. And because his crime and escape happened long before DNA analysis became a standard feature of police work - and because no close kin remain - investigators say they do not have tools that would aid a modern-day investigation.

The search for McGrath has stretched from coast to coast, to spots as far flung as Delmar, N.Y., Sacramento, and London, Ky. Hunter has investigated unidentified bodies in morgues, but the hunt continues because something is always off - dental records don’t match; the age, weight, or height is wrong; the hair color a shade off.

“He could be in a pauper’s grave in Cleveland, Ohio, or a popular businessman in California, or anywhere in between,’’ said Lieutenant Barry Hunter of the Sullivan County Sheriff’s Department, who as a state trooper in 1984 embarked on a quest to capture McGrath. “The fact of the matter is, we don’t have any idea where John is.’’

And then, on Aug. 13, 1974, 10 years into his life sentence, McGrath vanished. He has been a fugitive ever since.

But John William McGrath was also known as the homicidal patient who suffered a psychotic break from reality and shot his two younger brothers, mother, and father in the face with a high-powered rifle, one by one. A grand jury found him “insane and mentally deranged,’’ and a judge committed him to life in the state hospital.

NEWPORT, N.H. - The last time anyone saw inmate Number 9608 was 37 years ago on the lush lawns of the New Hampshire Hospital, where he was known as an avid writer, painter, and math wiz.

McGrath regularly wrote book reviews for the hospital newsletter. And he transformed the bland walls in the bowels of the facility into colorful murals of the New England countryside, with brooks and covered bridges. He completed correspondence courses in math and science and was given the responsibility of driving heavy equipment around the ball field and golf course.

Over the next three years, McGrath created a life for himself in the state hospital. He responded well to intense group, individual, and drug therapies, according to court documents, gaining freedom to walk the grounds unattended and live in a minimally secured ward.

But, according to court records, they said it would be “dangerous to permit the said John William McGrath to go at large.’’

McGrath was tried as an adult. But after prosecutors presented their evidence, including psychiatric evaluations that said McGrath “was suffering from a psychotic illness and should be indefinitely committed,’’ a grand jury in 1965 declined to bring charges against him, saying he was insane at the time of the murders.

McGrath then drove 45 miles to the state hospital in Concord, where he had been an outpatient for more than a year. He confessed to the night nurse and asked for a doctor. The front desk staff was reluctant to believe him until someone noticed blood on his clothes.

His mother, Willena McGrath, was the first to open the door. He shot her once. The force of the gunshot spun her around, and she landed face down in the snow. His father, Francis L. McGrath Jr., a World War II veteran, watched his wife’s violent death and ran. But his eldest son caught him and opened fire.

Next, McGrath walked to where his 5-year-old brother, Charles, slept, and fired. Then he waited. It would be nearly two hours until his parents returned home.

Sometime around 9 p.m. he grabbed the rifle he’d used to hunt deer, the rifle his uncle took from a dead Japanese solider during World War II, and shot his 14-year-old brother Peter twice, once in the elbow, once in the face.

McGrath murdered his family March 10, 1962. There was snow on the ground, and his parents had gone out. He resented babysitting, detested his parents’ bickering, feared his father, and was tired of his ambivalent relationship with his mother.

“It was a Saturday night, a pretty peaceful one. I loaded the gun, shot at my brother, then at the other one who was asleep. I felt I had done something wrong, that I shouldn’t have done it - but since I had started I should finish. I felt elated, in a state of shock, in a frenzy . . . but I had no emotional feeling. I was washed out.’’

His condition improved so much that in March 1969, the courts granted him conditional parole. Dr. Warren W. Burns, the hospital’s superintendent, testified that McGrath had gained significant self-awareness, recognizing his mood swings and alerting hospital staff so they could adjust his medications.

McGrath was allowed to take computer courses at the New Hampshire Technical Institute, provided he was escorted to and from campus and remained supervised for the hours he was not on hospital grounds.

But even limited freedom proved too much of a temptress, and within eight months, the new, acting hospital superintendent asked the courts to transfer McGrath to prison. Not only was McGrath getting illicit drugs from outside, but he helped burglarize the hospital pharmacy, court records show.

“He is uncooperative, unmotivated as well as incorrigible,’’ Dr. Christos Koutras, the acting superintendent, wrote in his transfer request. “He is a disrupting influence. He is also a danger to other patients as well as himself.’’

McGrath stayed in the New Hampshire State Prison until 1972, when he returned to the hospital. Court records said within a year, “the earlier problem occasioned by the illicit use of drugs has been completely eliminated.’’ He was moved out of the maximum security ward and granted supervised walks of the grounds.

Fifteen months later, McGrath escaped.

That was 37 years ago.

There have been blips on the radar, but the hits never turn out to be McGrath, whose fingerprints and dental records have been filed in a national crime database since 1988. But unlike fingerprints or DNA, dental records can change over time. Fillings can be replaced. Teeth can be removed.

“Oh gosh, I haven’t got a hit since the ’80s,’’ said Hunter, who has scoured records stored in attics and unearthed dental X-rays lost behind file cabinets in the hunt for McGrath. “I think the ability to identify bodies since then has improved a great deal.’’

But DNA is not an option. McGrath fled before the courts mandated inmates provide DNA samples, and none of his surviving relatives is of close enough relation to build a DNA profile, Hunter said.

There have been inquiries from “America’s Most Wanted’’ and numerous articles in local papers, which have generated tips but no credible leads. The US Marshals Service is looking for McGrath, too.

“I drive by the cemetery where his parents and two brothers are buried almost every single day,’’ said Hunter, who checks the gravesite on the anniversary of the deaths and on holidays, looking for clues - letters, fresh flowers, cleared debris. “I’m constantly reminded by people.’’

And as if to underscore this sentiment, a conversation about McGrath’s life on the run starts while Hunter makes copies at the Newport police department. Captain Robert Ballou’s father was McGrath’s high school math teacher.

“I can’t believe someone can disappear in this day and age,’’ mused Ballou, a police officer for 35 years.

“Let’s face it,’’ Hunter replied, “the smart criminal doesn’t involve anyone else. The smart criminal doesn’t tell anyone else.’’

As Hunter said earlier, “Had Whitey Bulger not trusted someone, he’d still be a free man.’’

There is one person McGrath may have trusted, a woman who remained fiercely loyal to him until the day she died in 1988: Delia Clark, his maternal grandmother.

He had a bedroom at her small two-story house when he was growing up, and she kept it as it had been even after the murders. Clark named McGrath as the sole heir of her $32,000 estate.

If she knew where McGrath was, it was a secret Clark never shared. It was also one taken to her grave, a place she now shares with her daughter, grandsons, and son-in-law.

Akilah Johnson can be reached at ajohnson@globe.com.

© Copyright 2011 Globe Newspaper Company.