The members of the class of 1983 slapped on their 40th reunion name tags and hit the open bar to reminisce. They squeezed into Johnny McGorey’s Pub across from the Massapequa Park train station on Long Island last weekend, among them former athletes, prom sweethearts and yearbook fixtures.

But the buzz of the reunion was the phantom of Berner High School: Rex Heuermann had suddenly made a name for himself days before, when prosecutors accused him of being a serial killer.

Mr. Heuermann, a 59-year-old architect and high school nobody who had lived his life several blocks from the bar, was arrested July 13. He was charged with killing three women found buried in 2010 near Gilgo Beach on Long Island’s South Shore, and is the prime suspect in the death of a fourth. In all, 11 bodies have been found on the miles-long stretch of shore.

Mr. Heuermann has pleaded not guilty, but his arrest created a sensation, drawing crowds to watch investigators carry evidence from his dilapidated ranch house. Ranks of television cameras and even drones recorded the excavation of the backyard. The news loop on the bar’s television screens broadcast continuous images of the man who had been a reclusive teen at Berner’s margins, sidestepping hallway society, a stranger to the cool-kid cafeteria tables.

At Johnny McGorey’s, the old classmates assessed each other, their inevitable hair loss and weight gain, and then measured how Mr. Heuermann had held up. His face had hardened, they observed, and his gawky frame was now hulking. The goofy glasses were gone, but he still had that mop of hair.

Classmates who remembered Mr. Heuermann, who graduated two grades ahead of them, described him as a victim, albeit one with a mean streak, whose home life was difficult and school life was worse. He was a loner, they said, and a target.

“He was everybody’s punching bag,” said John Parisi, who said Mr. Heuermann never really fell into cliques like jocks, nerds or burnouts.

“He got picked on a lot,” Mr. Parisi said. “He would take it and take it and walk away. I seen him pushed to his limit.”

In sixth grade, a group of students had “singled out” the tall, awkward boy and tried to beat him up. After being stopped by a teacher, they tortured him verbally. But in high school, Mr. Heuermann grew larger and more menacing, said Mr. Parisi, who graduated in 1983.

“I was really scared of him. He was the type of guy if he snapped he could really hurt you,” Mr. Parisi said. “He was disillusioned and he was misguided. You had to be very careful.”

Mr. Heuermann came of age in a New York suburb laid out on a tight grid of streets an hour from Midtown Manhattan, with measures of celebrity and notoriety alike. It spawned the acting Baldwin brothers and Jerry Seinfeld, but also Joey Buttafuoco, the swaggering auto body guy whose teenage mistress, Amy Fisher, shot his wife, Mary Jo, at her front door in 1992. Notorious crimes became part of the local lore, including serial killers like the Son of Sam who terrorized nearby Queens, and Joel Rifkin, who went to high school several towns away in East Meadow.

Berner High School is a squat, tan brick structure on the edge of town that students 40 years ago reached by Schwinn or Camaro. Its social groups had rituals: The jocks took trips to the beach and hung out at All American Hamburger Drive-In. The burnouts had Zappa Woods, a leafy hideaway where they could smoke weed and blast Led Zeppelin and the Doors.

Mr. Heuermann, unathletic and uncool, remained “an outcast,” said Dan Musto, 55, who said he knew him growing up. Mr. Heuermann did join the drama club as a stagehand. In a yearbook photo, he towers in the back row above the rest of the students, looking shy in large-framed glasses with his hair unstyled in a world of perfect feathering.

And after commencement on the Berner Bison football field, Mr. Heuermann remained removed from the alumni groups, reunions and eventual social media pages for graduates, even as bodies kept turning up on Gilgo Beach.

Over the past dozen years, the murders riveted locals. After numerous dead ends, investigators began closing in on Mr. Heuermann last year with the help of DNA analysis, cellphone records and a witness’s account of seeing a Chevrolet Avalanche like Mr. Heuermann’s.

For some last weekend, it was hard to reconcile the gawky, shy kid on the margins with the man authorities call a sadistic serial killer who preyed on women who worked as escorts. They say he wrapped them in hunting burlap, dropping them along a desolate stretch of Ocean Parkway a short drive from this very reunion, and within walking distance of the school’s favorite beach, Tobay.

“It’s a shock. We knew him,” said Michael Sean Fagan, speaking above a room filled with animated conversation and blaring retro soundtrack. “He was nerdy, smart.”

Others said the arrest made a piece fall into place.

“When I heard they arrested him, I was not surprised at all,” Don Ophals, who attended kindergarten through 12th grade with Mr. Heuermann, said in a telphone interview. “I said, ‘Oh my god, it fits perfectly.’ That’s the weird guy.”

“He was a recluse, very quiet,” said Mr. Ophals, a champion wrestler in high school and now a healthcare executive. “You just saw him as a guy by himself. He barely spoke.”

“He was seen as weird, someone you didn’t see eye to eye with.”

In another phone interview, the actor Billy Baldwin, who starred in “Backdraft,” said he had attended junior high and high school with Mr. Heuermann and had known him in passing, to say hello in the hallways and in shared classes. Mr. Baldwin also said Mr. Heuermann never fit into clearly established cliques, “but I also didn’t think he was so weird, so creepy or so unusual that it would lead to something like this.”

“He was a bit shy, a bit insecure, a bit uncomfortable,” he said. “I wouldn’t say he was an outcast but he struggled to fit in and to find his crowd.”

The struggle started early. Mr. Heuermann grew up with three older sisters and a younger brother. His father, Ted, was an aerospace engineer who enjoyed specialty woodworking, a hobby the adult Mr. Heuermann would emulate, making furniture in his garage.

But according to Mr. Musto, it was well known that Mr. Heuermann had clashed with his father, who was tough on the boy for not being a go getter. In response, Rex acted out. He got caught after engaging in a shoplifting spree, Mr. Musto said.

“Why is he getting in trouble? He’s fighting with his dad,” Mr. Musto said. “It was common knowledge.”

His father died when Mr. Heuermann was on the cusp of adolescence. It was 1975, when Rex was 12. After that, the children were raised by their mother, Dolores, now 93 and living in upstate New York.

Mr. Ophals said that in grade school, he once fought Rex simply because his older brother told him to, and prevailed easily. Back then, Mr. Ophals said, bullying was not monitored as it is today.

“That was just how it was at that time,” Mr. Ophals said. “You played the cards you were dealt.”

John DeMicoli, who grew up near the Heuermann’s rundown home on First Avenue, said young Rex preferred to remain at home, and essentially opted out of social life.

One thing Rex enjoyed was architectural drawing class, he said, “but when classmates would try to talk to him, he didn’t have the social skills to hold a conversation — just a very weird character.”

He was also known for fighting back after he was pushed past his limit.

“He had a mean streak in him,” Mr. DeMicoli said.

After graduating, Mr. Heuermann spent several years doing part-time cleaning and maintenance at Jones Beach, which is several miles west of Gilgo Beach, and also frequented Tobay Beach, which lies between.

Mr. Baldwin, who worked several summers as a Tobay lifeguard, said he saw Mr. Heuermann there periodically. Mr. Baldwin called it “very disturbing and ironic” that Mr. Heuermann had been charged with “burying bodies in the dunes, just walking distance from my lifeguard stand.”

Mr. Heuermann went on to college at New York Institute of Technology on Long Island to study architectural technology. He eventually started his own business in Manhattan as an architectural consultant and became proficient at making sure renovations followed intricate building codes — tormenting many of the contractors and homeowners he dealt with.

As a married father, he bought his family home in the 1990s and let it fall into disrepair as surrounding properties soared in value and were renovated.

One of the few neighbors Mr. Heuermann spoke to was Etienne de Villiers, 68, whose immaculately kept house next door stood in keen contrast with Mr. Heuermann’s. Mr. de Villiers said he had only passing conversations with Mr. Heuermann along with a few minor conflicts, like the time he had to tell Mr. Heuermann to stop leering at his wife over the backyard fence while she was sunbathing.

Mr. de Villiers watched as Mr. Heuermann seemed to be raising his children to be as isolated as he had been, in the same rundown off-limits house. He said that when Mr. Heuermann’s daughter Victoria, now 26, got her license, “I wanted to tell her, ‘Just get in your car and drive and never come back.’”

At Johnny McGorey’s Pub, Mr. DeMicoli was more concerned with raising a glass with former classmates than dwelling on Mr. Heuermann. But he recalled once when he and his friends had tried to the recruit the huge, awkward boy into their street hockey game. “He would have been a great goalie,” he said, almost wistfully.

The brief effort at inclusion came to nothing.

“He just didn’t want any part of it, he didn’t want any part of sports,” Mr. DeMicoli said. “He didn’t want any part of anything.”

Andy Newman contributed reporting.