One by one they left the Wildwood Acres Pavilion and headed up Hulse Avenue Monday night, lit candles in hand.

Braving temperatures well below freezing, approximately 70 friends and neighbors walked the few short blocks to the home on 12th Street where Tanya and Danielle Lawrence had been killed nearly 24 hours earlier.

The 43-year-old mother and her 17-year-old daughter were shot dead by Ms. Lawrence’s boyfriend, 44-year-old Thomas Calhoun, who then turned the gun on himself. Danielle was most likely shot coming to the aide of her injured mother, investigators said.

A younger daughter, Brandy Lawrence, 14, called 911 from inside the home at about 10 p.m. Sunday to report that her mother had been shot. She then successfully escaped the house before Mr. Calhoun killed her sister and himself, according to Det. Lt. Kevin Beyrer of the Suffolk County Homicide Squad.

The double murder and suicide were the tragic culmination of what police called a long breakup between Ms. Lawrence and Mr. Calhoun.

“That seems to be what started this,” Det. Lt. Beyrer said. “They were in the process of breaking up. It was a long process, but that’s how the argument started [Sunday night].”

Bob Greenberg, who lives across the street, said Monday morning that he had “heard arguments [at the Lawrence house] from time to time” since Mr. Calhoun had moved in.

Pieter Williams, a freshman at Riverhead High School who was visiting the Greenberg house Sunday night, said he heard the argument begin and then the gunshots. He said he would never have expected the incident to take a fatal turn.

“I was about 50 feet away,” Pieter said.

Riverhead Town and New York State police quickly arrived at the scene of the shooting and urged neighbors to get inside their houses.

Approximately two hours later, after more than a dozen law enforcement vehicles had swarmed the area around the house and blocked nearby roadways to keep others out of harm’s way, police entered the home.

Inside they found multiple guns — though police declined to say how many — and the three bodies. Det. Lt. Beyrer said it’s unclear exactly when Mr. Calhoun shot himself and that police were still trying to ascertain exactly which guns were used in the shootings.

The murders left the Wading River and Riverhead communities reeling.

“She shouldn’t have died the way she did,” said Amaya Coach, a classmate of Danielle’s at Riverhead High School. Saying she’s known Danielle since third grade, Amaya broke down in front of the group that gathered at the pavilion Monday night.

Danielle, whose father, Tony Lawrence, is a longtime physical education teacher and former lacrosse coach in Riverhead, was described as “kind, sweet and genuine” by her teachers.

“[She] always had a smile on her face,” said orchestra teacher Marisa Macchio. “During lessons, she would be helping others, motivated to learn and happy to be playing her instrument. In class, she was respectful and engaged in our rehearsals.”

Danielle played the violin in orchestra and was a member of the Blue Masques, Riverhead’s drama club. She was also actively involved in the Riverhead Community Awareness Program, serving as a peer adviser.

“She was the type of student that you wish you can have 10 of them just like her in your class,” said math teacher Athena Angerhauser.

Riverhead Superintendent Nancy Carney said counselors will be available to students as they cope with the tragedy.

“I am devastated for the Lawrence family and mourn their losses,” Ms. Carney said. “Danielle had her whole life in front of her and on behalf of the entire district, I want to express our deep sadness to the Lawrence family.”

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Classmates described Danielle as soft-spoken and shared stories during Monday’s vigil about how they admired the way she put other people’s needs ahead of her own.

“She was really quiet but worked hard to get positive messages across to kids,” said Ashleigh Rubino, a classmate and fellow CAP peer adviser.

Freshman Jesse Greenberg, Danielle’s neighbor, organized the memorial along with his friend Pieter Williams because they believed it provided an opportunity for themselves and the community to heal.

Danielle had recently expressed an interest in volunteering to help out at the next Community Arts Program show at East End Disability Associates, where her mother worked, according to Ms. Lawrence’s colleague there, Matthew Kuriloff. The program works with individuals with disabilities to teach them the performing arts.

Ms. Lawrence had started with the nonprofit as a direct service professional in 2011, working at one of its day program centers before taking a brief hiatus. She returned to work at East End Disability Associates about eight months ago as a case worker, helping individuals and their families seek and coordinate services available to them through Medicaid. She was a case worker for about 30 different clients.

“Between all of the people she worked with every day [in her previous role] who still knew her, and all of the individuals and families she was the point person for as a case worker, there were hundreds of people through EEDA she had a positive effect on in some way,” Mr. Kuriloff said.

Ms. Lawrence and her former husband were divorced in October 2013.

Mr. Greenberg said he was friendly with Mr. Lawrence when he lived in the house, but “didn’t even know” Mr. Calhoun’s name. His son said he had met Mr. Calhoun only once.

“I was just asking his name, actually,” Jesse recalled. “I was saying hi because I knew Mr. Lawrence and I was like, ‘Hi, my name is Jesse.’ He was like, ‘Hi, get out of here.’

“He was a bad guy. I had a feeling he was,” Jesse said.

Court records show that Mr. Calhoun had his share of run-ins with the law. He was convicted eight times in Riverhead Town Justice Court in the 1990s on charges ranging from disorderly conduct to driving while intoxicated, driving while ability impaired and harassment.

News-Review archives show he had been arrested at least twice more in the past 15 years — for DWAI in 2000 and DWI in 2011.

In 2005, he struck and seriously injured a 69-year-old woman who was crossing East Main Street in Riverhead, but no charges were filed in that case.

In 2011, while employed as a security guard for a private community in Westhampton, Mr. Calhoun filed for bankruptcy.

Jesse Swenk, who lives on nearby Park Street, also acknowledged that domestic incidents had been ongoing at the house where all three bodies were discovered Sunday night. He said Mr. Lawrence — whom he described as “a great guy” — had been having issues with Mr. Calhoun since September.

“One of the last times I saw [Mr. Lawrence], he was saying there was an issue at the house,” Mr. Swenk said. “He was trying to get the cops involved. It’s a mess.”

At the vigil Monday night, Pieter read a prayer after the crowd lit their candles. Jesse nailed a wooden cross to a tree. Candles, teddy bears and notes were left in the snow lining the Lawrence property.

Classmate Brooke Beier said she’ll miss Danielle’s smile.

“I want to show support for her and her family,” she said before joining the candlelit march. “She was very kind-hearted and always wanted to help others.”

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Editor’s Note: This story was most recently updated Tuesday, Feb. 17 at 10 p.m. to reflect the most recent details.