The deadliest day in Syracuse police history in over 30 years began when two officers attempted to pull over a gray Honda Civic on a quiet corner outside a church. The car peeled away, out of sight — but not before the police officers clocked its license plate.

The officers soon tracked the Civic to Darien Drive in the town of Salina, a suburb about 15 minutes away. It was parked in front of a well-kept house. But when an officer approached and looked inside the vehicle, he saw the first sign of real trouble, and reached for his police radio.

“There are AR-15 magazines in the back of his car,” he said, and described the sounds of a gun being loaded: “We hear racking coming from the house.”

The street had settled into its quiet Sunday night rhythms. Duane Shenandoah, 80, was out back nearby with his daughter, a wedding planner, helping her paint a backdrop for an upcoming ceremony. Another neighbor, Mousa Alzokari, 47, who works at a Syracuse newsstand, was home with his five children in their living room. And across the street, Daniel Kay, 73, was reading his Sunday newspaper at his front window.

Onondaga County sheriff’s deputies met the officers outside the house with the Civic, and they made a quick plan. One of the deputies, Lt. Michael Hoosock, 37, hurried around toward the back. Tucking behind a neighbor’s maple tree for cover, he watched the house in question. Two Syracuse police officers watched the front door.

The authorities would spend hours piecing together the moments that followed.

Behind the house, in the darkening night, a form crept onto a back deck: a man with an assault rifle.

Reading his paper across the street, Mr. Kay heard popping sounds and rose to his front door. He called to his wife: “Who the hell is shooting off all these fireworks at this time of night?”

The pops quickly became deafening, stretching into a long and terrifying series. Neighbors ducked for cover in their homes. A bullet punched through a wall in Mr. Alzokari’s living room where his family had just been sitting.

Lieutenant Hoosock, a bomb-squad officer with three children at home under the age of 8, was hit as he tucked behind the tree. He wasn’t able to return fire — and was probably unaware he was even a target.

The form on the deck then moved toward the front of the house, and the assault rifle fired again. Police Officer Michael Jensen, 29, with just two-plus years on the job, was hit, but he was able to return fire, as was his partner.

There was bedlam on the street: Mr. Shenandoah, helping paint the wedding backdrop, heard the shots and smelled the acrid odor the rifle gave off. Officers burst through the back door of Mr. Alzokari’s house and hurried upstairs to a window facing the shootout, for a clear vantage.

The man with the assault rifle, later identified as Christopher Murphy, 33, the oldest of five siblings raised in that house on Darien Drive, was hit and went down.

The shooting stopped, leaving three fallen men. All would die from their injuries. An officer with a bullhorn ordered anyone else in the home to come out. There was no reply.

Mr. Alzokari looked in disbelief at the bullet hole in his wall. He was an immigrant from Yemen and had lived in Syracuse for 19 years. The Murphy family had always made him feel welcome.

Slowly, details from the hours leading up to the bloodshed would reveal themselves.

Mr. Murphy was born in 1990, followed by triplet brothers and a sister. Together, the siblings were fixtures on Darien Drive. All attended Liverpool High School, where they mostly excelled: Their parents, strict but doting, would accept nothing less than a B average.

The brothers were friends with other boys in the neighborhood — Shawn Kinsella and his siblings around the corner, and NaKeem Williams and his brother, the whole group often found in someone’s den or on the school basketball court. As adults, they all gathered to play darts, pool and cards at local watering holes like Cobblestone Ale House, the Retreat and Endzone Bar & Grill.

Mr. Murphy seemed particularly close to Mr. Kinsella, a gentle, towering presence, leaning on him when something went wrong.

Mr. Williams said Mr. Murphy was responsible and smart, with a mind for mechanics, so it was no surprise that he had landed a steady, well-paying job at JMA Wireless, an internet company based in Syracuse. But trouble emerged, most notably a cocaine habit that had seemed, to old friends, to have gone from recreational to something more.

On Sunday, he had been drinking at the Endzone, where he met a friend and then left, and at some point began using cocaine, according to the Onondaga County district attorney, William J. Fitzpatrick. Mr. Murphy’s erratic driving seems to have been what led to the attempted traffic stop, in a neighborhood called Tipperary Hill, some five miles from his home, that launched the deadly chain of events at around 7 p.m.

After Mr. Murphy sped home, his friend became uncomfortable with his behavior, called a car service and left. Mr. Murphy then left his house, picked up Mr. Kinsella — the old friend he relied on in hard times — and returned home, Mr. Fitzpatrick said.

When the officers arrived at Darien Drive around 8:30 p.m., Mr. Kinsella saw Mr. Murphy arm himself with an assault rifle. Mr. Murphy urged Mr. Kinsella to flee, and he did, through the back door and over the fence, the police said.

There were no interactions between the gunman and the police before Mr. Murphy ambushed the deputy in the neighboring backyard, the authorities said.

Officials said the last time a police officer had been fatally shot in the line of duty in Syracuse was in 1990. An Onondaga County sheriff’s deputy was killed in 2003 when he was struck by a passing vehicle while helping a stranded motorist.

On Tuesday, the Murphy house remained an active crime scene, as investigators sought clues from within and outside. Mr. Kinsella was taken in for questioning and was cooperating, the police said.

Among the unknowns was how many shots had been fired with the assault rifle. At a news conference on Monday, Sheriff Tobias Shelley said that number was “too difficult to count.”

And, of course, there was the question of motive. That remained, even among Mr. Murphy’s circle of old friends, a mystery. They had never heard Mr. Murphy speak ill of law enforcement. “In my 32 years on this earth, I would never think he would do this. Ever,” Mr. Williams said. “There’s something we’re all missing to this story. There has to be.”

Chief Joseph L. Cecile of the Syracuse Police Department, visibly drained 20 hours after the shooting, offered reporters the smallest glimpse into the authorities’ thinking.

“There’s something in his past that I don’t think we’re going to reveal at this point,” he said, “that may be some indicator as to why he did it.”

Cole Louison contributed reporting.