In his office near the Empire State Building, Rex Heuermann was a master of the meticulous: a veteran architectural consultant and a self-described expert at navigating the intricacies of New York City’s building code. He impressed some clients and drove others crazy with his fine-toothed directives.

At his home in Massapequa Park on Long Island, while some neighbors saw Mr. Heuermann as just another passenger in a suit, others saw him as a threatening figure. He scowled at the neighbors as he brandished an ax in the front yard of a dilapidated house that parents warned their children to avoid on Halloween. He got kicked out of Whole Foods for stealing fruit.

“We would cross the street,” said Nicholas Ferchaw, 24, a neighbor. “It was someone you didn’t want to get close to.”

On Friday, Suffolk County prosecutors said Massapequa Park residents had a serial killer living among them. They accused Mr. Heuermann, 59, of leaving a quarter-mile trail of bodies of young women on the south shore of Long Island in what became known as the Gilgo Beach murders. However, he was so careful to cover his tracks, they said, that it took nearly 15 years to arrest him.

Mr. Heuermann’s friends and clients in the real estate business were stunned.

Her neighbor Mr Ferchaw said: “I wasn’t surprised at all, because of all the creepiness.”

Mr. Heuermann, who was arrested in Midtown Thursday night, was charged Friday with three counts of first-degree murder and ordered held without bail during a brief court appearance in Suffolk County. His lawyer said outside court that Heuermann denied committing the murders.

If convicted of these crimes, Mr. Heuermann would join the ranks of serial killers who lead double lives, the other quite mundane. John Wayne Gacy was a construction contractor in Illinois. Richard Cottingham, known as the Torso Killer, was a computer operator for a New Jersey insurance company.

in a video interview posted on YouTube last year and carried out in his completely normal-looking office on Fifth Avenue, Mr. Heuermann, tall and beefy, sporting a 1970s toupee haircut and a blue dress shirt with a pen sticking out of the pocket, comes across as a recognizable character: the feisty, street-smart, I-got-a-guy, Noo Yawker.

“When a job that should have been routine suddenly isn’t routine,” he tells the interviewer, Antoine Amira, “I get the phone call.”

According to his resume and his company website, HR Consultants and AssociatesMr. Heuermann’s clients included American Airlines, Catholic Charities and the city’s own Department of Environmental Protection. He has represented clients before the Landmarks Preservation Commission many times and claimed credit for hundreds of successful applications with city agencies.

Steve Kramberg, a property manager in Brooklyn who has worked with Heuermann for about 30 years, called him “a gem to deal with, very knowledgeable.” Heuermann was “a big, goofy guy, a bit of a nerd” who worked long hours and was available around the clock, Kramberg said. But he was also devoted to his wife, whom Kramberg said he had health problems, and to his elderly mother.

In Massapequa Park, a tightly gridded town of neat houses with manicured lawns, Mr. Heuermann, the son of an aerospace engineer, lived in the house he grew up in and fixed furniture in his father’s old workshop. A man who went to high school with him said he was bullied as a teenager but sometimes stood up for himself. In 1990 he married an executive from an office supply company. She has a daughter who works in her company.

Mr. Ferchaw recounted several confrontations with his neighbor, none of them pleasant. There was a moment when he waved at Mr. Heuermann while he was chopping wood and Mr. Heuermann responded with a silent look back between slices of his mallet. He other times he was sitting next to his stacked firewood on the porch watching an old television.

Mike Schmidt, who has lived in the neighborhood for 10 years, has a friend who lives behind Mr. Heuermann. Sometimes Mr. Schmidt would visit his friend, have a few beers in the backyard, look at the Heuermanns’ ramshackle house, “and say, ‘He probably has bodies in there.’”

This past Halloween, Mr. Schmidt and his friend decided to take their children trick-or-treating at Mr. Heuermann’s house, just to see inside. They were surprised when Mr. Heuermann himself answered the door and gave each child a small plastic pumpkin filled with candy.

When Mr. Schmidt’s wife found out where the candy came from, she made him throw it away.

At work, Heuermann’s meticulous approach annoyed some people. Kelly Parisi, former president of the cooperative board of a Brooklyn Heights building that hired Heuermann to oversee the renovations, said he was “contradictory with everyone” and so “too annoying” that the board eventually fired him.

Paul Teitelbaum, another former chair of the building’s board, described him as “a really cold and distant person, kind of creepy.” He added: “There was an arrogance: ‘I’m the expert, you’re lucky to have me’.”

But one man’s arrogant demands were another’s eye for detail. “He was very good at guiding things,” Kramberg said.

According to the schedule released by prosecutors and the Department of Buildings and court records, Mr. Heuermann kept up his busy work schedule even as victims disappeared.

In 2009, prosecutors said, after killing Melissa Barthelemy, a 24-year-old who worked as an escort, Heuermann made a series of taunting calls to her family, during lunchtime and after-hours, from nearby locations. to his office.

In June 2010, about two weeks after Megan Waterman, a 22-year-old from Maine, was last seen alive, Mr. Heuermann filed an application to install a new fire escape in a building in Cobble Hill. , Brooklyn. In August of that year, he filed an application to repair the terracotta and grout the bricks on a building on the Upper West Side, nine days before Amber Lynn Costello, 27, went missing near her home a few miles from the city. from Heuermann.

On March 9, 2022, as the investigative raid was tightening, Mr. Heuermann was writing a typically detailed letter to a lawyer about a project on West 71st Street:

“It appears from my traversal that the drain line is above the interior floor slab and if the trench drain is placed below this level it would not be able to drain by gravity,” he wrote. “I would highly recommend an investigation into the use of downside waterproofing at this site.”

Five days after that, investigators discovered that Mr. Heuermann had owned the same model of van that a witness said Ms. Costello’s killer had driven. Two weeks later, prosecutors said, Mr. Heuermann Googled “Long Island Serial Killer” and saw an article titled “New Task Force Aims to Crack Long Island Serial Killer Case.”

It was late last summer that Heuermann, sweaty and dressed in a dirty T-shirt and shorts, was spotted at a Massapequa Park Whole Foods stealing clementines from a kid’s bowl.

“He took three and put them in his pocket, then he took more,” said Tara Alonzo, a store clerk. After a few more rounds, she called out to him. “I said, ‘Sir, those are for the children,’” she recalled. She said Mr. Heuermann yelled back at her and became so enraged that her manager walked him out. She did not see her face again until she appeared on television on Friday.

“My coworker said, ‘That’s the orange guy!’”

Kramberg said he had spoken to Heuermann on the phone Thursday night. He was the same talkative as always, telling jokes.

“That must have been right before he left the office and was arrested,” Kramberg said.

ginia bellafante, corey kilgannon and michael wilson contributed reporting. jack begg contributed research.