MINNEAPOLIS — There was something bad going on in the alleyway behind the house, she told her fiancé on the phone, someone who sounded as if she was in distress, maybe a rape. It was past 11 p.m., and most people on Washburn Avenue were furled in their beds.

Except Justine Damond, alone at home with the noises, her anxiety creeping into the loud Las Vegas casino where her fiancé had answered the phone.

They had met five years ago, when they lived 9,000 miles apart, beginning a courtship at first halting and then headlong. Now the wedding dress was ordered, the suit bought, the invitations sent, the ceremony set for an August weekend in Hawaii. But last Saturday night, they were separated again.

Her fiancé, Don Damond, told her to call 911. They stayed on the phone until she said the police had arrived. Stay put, he told her. Call me back, he told her.

“I have played this over in my head over and over,” Mr. Damond said on Friday in his first interview since that night. “Why didn’t I stay on the phone with her?”

The events of the next few minutes will be anatomized and argued over and, maybe, at some point, contested in court. But this much is established: As the squad car she had summoned slid down the alley, Justine Damond went up to the police officers inside, one of whom, for reasons still unknown, fired his gun, hit her in the abdomen and killed her.

Even to Americans now used to dissecting police shootings, the circumstances were an odd jolt: a black Somali-American cop, firing at a white Australian woman among the garages and green compost bins of an unremarkable strip of Midwestern concrete.

In Australia, where Ms. Damond, 40, grew up, there was agony and disbelief, the prime minister voicing bafflement, the tabloids in full cry. In the United States, there were questions about the officer’s failure to turn on his body camera, about firearms procedures, and about the role race has played in how officials responded. On Friday, the Minneapolis police chief was forced to resign.

And in interviews this past week in Sydney and Minneapolis, Ms. Damond’s friends and her fiancé were trying to fill in the blanks of her final night.

A week has passed. A cardboard sign at the end of the alleyway, propped amid the flowers laid there by friends and neighbors, asks the still-unanswered question: Why?

From Silence to ‘Yes’

She was the luminous Australian in the Fulton neighborhood of Minneapolis, leading meditation sessions, scattering her communications with rainbow emojis and greeting people with, “Hello, beautiful!”

One moment her friends remember her for is the time she rescued a flock of ducklings from a street drain, descending barefoot to scoop them up. It was only last month.

At one of the talks she occasionally gave at the Lake Harriet Spiritual Community, Ms. Damond, again barefoot, told the story: “You’ve never lived until you’ve had eight ducklings fling themselves into your lap because they’ve realized you’re trying to help,” she said. “So beautiful!”

Born in pre-revolutionary Iran to an Australian mother and an American father who was teaching English in Tehran, Justine Ruszczyk grew up on Sydney’s North Shore with an affinity for horses, a three-legged dog named Brad and any animal she could rescue and nurse to health in her home. Her mother was a nurse midwife; her father owned a bookstore.

She studied to become a veterinarian, but disliked that so much of the job consisted of spaying animals, said Sara Baldwin, her godmother. Then, when she was 22, her mother died of cancer. In pain and confusion, she went to an ashram, emerging from a three-week silent retreat with a determination to practice a different kind of healing.

“There was a time when I wasn’t sure I wanted to stay on the planet,” she wrote on her blog in August 2014. “It took me 13 years to come to where I am now — living with a deeply connected understanding of what it means to be a spiritual being in this very physical experience, a clear and grounded understanding of how this reality around me comes into being — and to be honest it was a pretty long and painful journey at times.”

She found what she was looking for in the teachings of Dr. Joe Dispenza, a chiropractor with a wide following for his ideas about changing lives through the power of the human brain. At a meditation retreat in Colorado Springs in 2012, she met Mr. Damond, a casino manager from Minneapolis.

“Hey, I just met my future wife,” he told a friend when he returned. “The only problem is, she lives 9,000 miles away.”

They chatted on Facebook for months, but when Mr. Damond declared his feelings for her, Ms. Ruszczyk went silent for more than a year. She told friends that she did not reciprocate until, having drawn up a list of the traits she wanted in a partner, she realized Mr. Damond was a match.

They met in Maui, in Australia, in San Francisco, impatient with happiness. The day he planned to propose, standing in the Marin Headlands with the Golden Gate Bridge in the distance, it was cold, and she kept trying to put her hand in his coat pocket where he had the ring. He had to keep grabbing her hand to stop her from finding it before he was ready to pull it out himself.

She said yes.

Reluctantly leaving Australia, Ms. Ruszczyk, who through her father was an American citizen, moved to Minneapolis in 2015.

“She had her family there,” Mr. Damond said on Friday. “All her friends, lifelong relationships, and she moved here for one person.”

Though she took his name, they put off marrying, partly because a wedding with families on two sides of the world would be hard to organize, partly because Ms. Damond was so absorbed in a new project, creating training materials for Mr. Dispenza.

Someday, she told Ms. Baldwin, she hoped she and Don would return to Sydney.

It was not only the weather or her friends or family that drew her back.

“She didn’t like the guns” in the United States, Ms. Baldwin said. “She didn’t like the violence.”

‘We Think It’s Justine’

With Mr. Damond in Las Vegas for work, she had been sleeping on his side of the bed — the left, under a pair of dream catchers — when she heard a scream for help.

She walked over the white shag rug to the windows that overlook the backyard. She peeked past the massive oak tree. The noise was coming from near a neighbor’s garage on the right, she told Mr. Damond.

At 11:27, a call came in to 911.

“Hi, I’m, I can hear someone out back and I, I’m not sure if she’s having sex or being raped,” Ms. Damond reported, according to a transcript released by the Minneapolis police.

“We’ve already got help on the way,” the operator promised.

Eight minutes later, officers had not arrived. Ms. Damond called back, wondering if they had gone to the wrong place. They were coming, the operator reassured her.

Nearby, Officer Matthew Harrity, with a year on the force, and Officer Mohamed Noor, with 21 months, got the call.

Officer Noor had been the first Somali cop in the immigrant-rich Fifth Precinct, his hiring hailed by the mayor and Minneapolis’s Somali community. He was supposed to be a bridge, leaping over the chasm of ingrained suspicion between the community and the police.

Here, now, he was another officer, less than three hours from the end of a 10-hour shift.

They turned their Ford Explorer into the alleyway behind Ms. Damond’s house, driving south along a stretch of concrete and asphalt wide enough only for one car. Their lights were off. Under the street lamps, the detached garages on either side were pale in their vinyl sidings.

As they reached the end of the alley, Officer Harrity, who was driving, was startled by a loud noise near the squad car, he told investigators. Then Ms. Damond came up to his open window.

Officer Noor fired.

Past his partner, through the window, the bullet found Ms. Damond’s abdomen. The officers got out of the car, calling back to the dispatch center, as the operator’s computer recorded the first sign that lives were about to change on two continents: “ONE DOWN … STARTING CPR.”

In Las Vegas, Mr. Damond’s texts to his fiancée were going unanswered. Maybe she had just gone back to bed, Mr. Damond thought.

Around 12:45 a.m., the Minneapolis police called. There had been a shooting. A woman had died.

Mr. Damond told himself it must have been the woman being raped.

Then he asked who the victim was.

“We can’t give a positive I.D., but we think it’s Justine,” Mr. Damond said he was told.

He sat at a slot machine, hyperventilating. Had the rapist killed his fiancée? He was already at the airport when the Minnesota Bureau of Criminal Apprehension, which is investigating the shooting, called to say that the person who had pulled the trigger was a police officer.

Mr. Damond got a seat on the 7 a.m. flight to Minneapolis. When he landed, a friend, a Delta Air Lines employee, was waiting. Hugging him, Mr. Damond heaved with sobs.

“There’s like a glitch in the matrix,” Mr. Damond recalled thinking. “ I just know I’m going to wake up from this nightmare.”

Outrage Down Under

On Monday afternoon in Sydney, news that a local woman had died overseas flowed into the offices of The Daily Telegraph, a tabloid that is part of the Rupert Murdoch news empire.

“It became very clear that it was the best story for us that day,” said the editor, Christopher Dore.

“You know, here’s this Aussie girl who goes over to find love,” he added. “And because of the complications of American policing and guns, she’s dead.”

The paper’s editors had a picture that they knew would pull heartstrings: Ms. Damond in a white blouse, smiling widely, her engagement ring sparkling on one hand.

At 6 p.m., a couple of hours to deadline, they began brainstorming headlines. They tried “Only in America,” but on the page, it looked off.

They settled on “AMERICAN NIGHTMARE.”

Mr. Dore insisted that the headline was not a judgment. “There’s no way in the world we’re going to lecture the United States about its Constitution or the right,” he said.

But the headline captured many Australians’ dismay over what, to them, seemed a peculiarly American phenomenon. There are tight restrictions on firearm ownership in Australia, and though police officers there carry guns, fatal police shootings are relatively rare.

“It would have never happened here,” said Michael Timbs, a mourner who showed up at a sunrise vigil at the beach near Ms. Damond’s childhood home on Wednesday.

That morning, Prime Minister Malcolm Turnbull appeared on television to ask, “How can a woman out in the street in her pajamas seeking assistance from police be shot like that?”

One Shot, Many Questions

A year after a police officer in a Twin Cities suburb fatally shot Philando Castile, a 32-year-old black driver whose dying moments were streamed by his girlfriend on Facebook, some of the same questions have pursued the shooting of Ms. Damond.

What led Officer Noor to fire his weapon? The loud noise the other officer said he had heard? Fear of an ambush, as his partner’s lawyer has implied?

At this point, almost everything is conjecture. Neither officer had his body camera turned on, leaving investigators and the public blind, a fact that the Minneapolis mayor, Betsy Hodges, has called “unacceptable.”

Officer Noor, whose record included three civilian complaints and a lawsuit over his treatment of a woman while performing a mental health checkup, has declined to speak with investigators.

Both officers have been placed on leave, and on Friday, the mayor forced the police chief, Janeé Harteau, to resign. It was an abrupt end to a contentious tenure, during which Chief Harteau faced criticism over her handling of other police shootings, including the killing of a black man, Jamar Clark, that led to weeks of protests. Activists have also questioned why officials moved so decisively in this case to condemn the shooting, compared with other police shootings in which the victims were black.

But as in other cases, prosecutors might find it difficult to make a case against Officer Noor if he argues that he believed he was in danger.

A 1989 Supreme Court decision, Graham v. Connor, held that officers’ actions had to be judged by whether force was reasonable given what the officer knew at the time.

“There is this huge misunderstanding in this country about the rules surrounding police officers’ use of deadly force,” said Jim Bueermann, a former Redlands, Calif., police chief who is now the president of the Police Foundation, a research group. “People just say, if a person was unarmed, why would an officer have shot him or her?”

In fast-moving situations, police protocol often leaves little room for error.

Officers usually have a round chambered in their sidearms. And experts say they are generally taught to draw their guns when they feel they or someone else are in imminent danger. Even for many traffic stops, officers keep a hand on the weapon while it is in the holster.

Mr. Bueermann said he believed many officers were quicker to pull their guns than they would have been a decade or two ago. “There is constant messaging to police officers about the dangers of their jobs,” he said. “There’s a really common adage in policing: It’s better to be tried by 12 than carried by six.”

He also questioned whether Officer Noor might have accidentally discharged his weapon — a far more common event than many realize, he said.

What made this shooting particularly bizarre, to veteran police officers, was that Officer Noor fired at close range past his partner. Many officers would be furious or unnerved if a partner shot across them in any situation short of being attacked, said Vernon J. Geberth, a former New York City police commander and the author of “Practical Homicide Investigation,” a widely used textbook.

The officer’s partner might well be thinking, “You could’ve shot my head off,” Mr. Geberth said.

Personal Effects

Barefoot, on a beach in Kona, Hawaii, exchanging vows under a wooden arch trellised with Tibetan prayer flags onto which their guests would add prayers of their own: That was the plan. The wedding was set for Aug. 17.

Mr. Damond would wear a bright blue suit with an open-collar white shirt. Following tradition, he had never seen Ms. Damond’s dress.

Now last-minute wedding tasks had given way to the business of death. There was a cremation to arrange, her clothes and engagement ring to pick up from investigators.

And Justine’s dress. On Sunday, he said, he was planning to go to the bridal shop, where he would see it for the first time.