Long after the sirens, vigils and cable news debates, the question remains.
It nags at survivors and their families. It haunts investigators as they comb through the gunman's belongings, text messages and the scattered pieces of his life.
Even as our attention as a society fades, the mystery of motive lingers like an open, forgotten wound until the next shooting, the next cycle of grief, outrage and desperate search for answers.
In Parkland, Florida, investigators have an unusual opportunity to answer that question after a high school massacre left 17 dead, because the suspected shooter remains alive - a rarity in these kinds of mass shootings.
This week, prosecutors said they would seek a death sentence for 19-year-old Nikolas Cruz, saying the rampage "was committed in a cold, calculated, and premeditated manner."
In coming months, Cruz - who police say confessed to the Feb. 14 shooting at Marjory Stoneman Douglas High School - will be scrutinised in granular detail by detectives, lawyers and forensic psychiatrists. All of them will try to answer some form of this question: Why?
"You want to do everything you can for the families, and that's the question that is being asked. Why my son? Why my daughter? Why was this person ripped from me for this senseless act?" said Daniel Oates, who was police chief in Aurora, Colorado, when a shooter in 2012 tossed tear-gas grenades into a movie theater and began firing indiscriminately into the fleeing crowd, killing 12. "We're all human beings; we strive for explanation."
Cruz was indicted last week on 17 counts of premeditated murder and 17 counts of attempted murder. As he appeared in court Wednesday for arraignment, tens of thousands of students were marching out of classrooms to decry gun violence, and far more are expected to gather next week for a protest in Washington.
Officials, however, have been unable to explain the motive behind last month's shooting. A portrait has emerged of Cruz as a troubled young man with a pattern of alleged violence and unnerving behavior. But when asked what may have driven him to violence, Broward County Sheriff Scott Israel has simply responded: "Pure evil. Pure evil."
Of the long list of mass shootings in recent years, Aurora's offers the best hints for what will come next in Florida.
Like Parkland's accused school shooter, the Aurora gunman, James Holmes, had no clear motive, showed signs of mental-health problems before his attack and, perhaps most relevant, he also survived to stand trial.
In 2014, after two years of legal motions and delays, a court assigned a forensic psychiatrist named William Reid to meet with Holmes for a series of interviews over the course of two months. The interviews were intended to assess Holmes's sanity and his state of mind leading up to and during the shooting, but they also presented perhaps the best chance anyone would have to figure out what exactly drove him - and what drives others - to commit such senseless, devastating rampages.
"The why isn't always a legal question," Reid said in a recent interview. "But of course, it's what everyone wants to know."
Aurora theatre gunman, James Holmes
Reid asked to have Holmes transferred to a quiet, empty unit of the Colorado Mental Health Institute for their first meeting. There would be no shackles or handcuffs on him. Nor any guards in the room. Reid said he arranged the furniture to sit slightly off-center across from Holmes so that Holmes could look him in the eye but also look away if he chose.
"I got you some juice," Reid told him as they sat down. "They said you like juice."
Over the course of nine meetings - more than 22 hours locked in with the accused killer - Reid probed Holmes on his childhood, his studies as a PhD student and virtually every aspect of Holmes's life leading up to the shooting.
In videos of those interviews, Holmes said he had thought of killing others for months, even years before. He said that as a child, he resented his family's move to San Diego - becoming so upset that he tried to cut his wrist with cardboard. He said he chose to study neuroscience partly in the hope of understanding his own struggles.
A breakup with his then-girlfriend months before the shooting upset him, Holmes said. He was worried he might commit suicide and believed he could prevent that by taking other people's lives.
He talked of a strange belief that took shape in his mind, in which he could earn a point for every person he killed. He wanted to gather those points, but he said he wanted to avoid earning points by killing children. (At the movie theatre, he ended up killing one child and wounding others.)
He described feeling an aversion to other people. Not a murderous rage, but a cold, dismissive hate. He hated others, he explained, the way some people hate broccoli.
Before he opened fire on July 20, 2012, Holmes had sketched out in chilling detail his plan of attack, Reid noted. Writing in a brown wire-bound notebook, he ruled out using bombs or biological agents as too complicated. He mapped out different theatres and decided to lock the doors to maximise casualties.
The notebook - much of it written in cold, calculating language - includes seven consecutive pages in which a single word is written over and over, growing larger and larger until the loopy cursive practically screams off the page: "WHY?" "WHY?" "WHY?"
Toward the end of their interviews, Reid asked Holmes what he believed caused him to kill others. Holmes boiled it down to hard numbers: 45 percent was caused by his belief in the point system, another 45 by the feeling it would prevent his suicide, and 10 percent by his broccoli-like hatred.
Reid, however, said he found those answers lacking.
"You could call them excuses in a way, because they don't make sense," he said. "People break up with their girlfriends every day; that doesn't mean they become killers. They struggle with depression and impulses; that doesn't mean they become killers. These things are associated with the action, but they are not predictive."
Oates, the former Aurora police chief, said he, too, thinks authorities never got a real explanation. In 2015, Holmes was found guilty and sentenced to life in prison without parole.
After all those gruelling sessions with Holmes, after writing an entire book on the matter to be published this year, Reid said he understands the public frustration with the lack of answers.
"We want to settle the question in our minds," he said. "We want to be able to explain this world because that makes it somehow safer, and we don't have to be afraid every time the lights go down in a theater, every morning that our kids leave for school. But the truth is that the answers aren't easy."
What kind of person would do this?
Megan Greene has found herself searching online at all hours for new clues since the Las Vegas shooting.
She hid behind a truck when the first shots rang out on October 1. As a survivor, she struggles with guilt and panics at the sound of fireworks and school bells. Therapy and anxiety medication have helped. But the unanswered question has continued eating away at her.
"I'm constantly asking: Why? What kind of person would do this?" said Greene, who lives in Simi Valley, California.
She found herself asking that in moments of silence, sitting alone in her house. For a while, she was obsessed with reading every account of the shooting, believing something hidden between the lines might explain the horror she witnessed.
Some mass killers leave little doubt to motive.
The avowed white supremacist who killed nine black parishioners inside a Charleston, South Carolina, church in 2015 bluntly laid out his racist reasons in writings and a confession.
But in many shootings, like the one in Las Vegas, answers are elusive.
Authorities say they still have no idea, after months of exhaustive investigation, what drove the Las Vegas gunman, Stephen Paddock, to kill 58 people. In desperation, police even sent the shooter's body and brain in recent weeks to neuropathologists at Stanford University. But they found nothing unusual aside from high blood pressure and bad teeth.
In the absence of definitive answers, experts who research mass shootings talk instead in terms of patterns.
In a 2004 study, a psychiatrist named Paul Mullen conducted in-depth interviews with five Australian mass shooters, who had killed more than 50 people among them. He found several overlapping characteristics: All described feeling that the world was persecuting them. They were rigid, socially isolated and obsessive. They tended to be bullied as children and fascinated with guns. All also exhibited signs of general depression, but not diagnosable clinical depression.
"Often these are people who have personality traits like anger, but they don't have diagnosable conditions that would allow you to commit them to a hospital," said Phillip Resnick, who served as a forensic psychiatrist in cases including the Oklahoma City bomber, Timothy McVeigh, and the Unabomber, Ted Kaczynski.
In the wake of such killings, there is often a rush to point to mental illness as an explanation, but Resnick and almost all psychiatrists say that knee-jerk reaction is wrong.
In a 2015 study that examined 235 people who committed or tried to commit mass killings, only 22 percent of them could be considered mentally ill. A different study two years ago, published in the journal Annals of Epidemiology, found that "the large majority of people with mental disorders do not engage in violence against others, and that most violent behavior is due to factors other than mental illness."
"We like to think that anyone who kills others is somehow mentally ill," Resnick said. "But you have to remember, people kill for all sorts of reasons. They kill for profit or love or greed."
'Going for the killing record'
More than almost anyone, Tom and Caren Teves understand obsessions with the "why."
But after seeing the face of their son's killer - the Aurora theatre gunman - paraded on TV and in the pages of newspapers for five years, they've come to resent the endless focus on the gunmen and the minute dissections of their lives. More than that, they fear the powerful force such attention exerts on future mass shooters.
"People always talk about how we have to figure out what drives these guys so we can stop the next one," said Tom Teves. "But one motivation every scientific study has found in common with them is their desire for infamy."
Over time, the search for motive, the couple argue, becomes the motive.
In Mullen's 2004 Australia study, for example, mass killers spoke chillingly of their desire for fame, with one saying he was "going for the record" in his violence. The men, Mullen concluded, craved a manner of death that would bring them fame coupled with the aura of power and evil.
"When we engage in an obsessive questioning of why a shooter did it, we are granting their exact wish," said Park Dietz, one of the country's foremost forensic psychiatrists.
Over the past three decades, Dietz has evaluated more than a dozen mass murderers, including the Charleston shooter Dylann Roof. And for Dietz, there is no mystery surrounding their motives.
The specifics may vary from case to case, he said, but the broad strokes always boil down to two elements: "First is the blaming of others for their own suffering or the suffering of someone they care about. The second is their quest for immortality."
While sitting through the Aurora shooter's trial, Caren Teves heard one comment from her son's killer that became seared into her memory: "He said he didn't think he could make a mark on the world with science, but he could become famous by blowing up people."
In the years since, the couple have formed a group called No Notoriety, to persuade news media not to overpublicise mass shooters. While the debates on gun control and mental health after such shootings are important, Caren Teves said, "This one factor is the easiest one to fix. It doesn't take an act of Congress. We just stop saying his name."
In recent years, she has stopped searching for a definitive motive. "You come to realise that the 'why' is not going to bring your son or mother or brother back," she said
Instead, she focuses on the last moments of her son's life. Five years ago, when the Aurora gunman began his massacre, Alex Teves, 24, lunged toward his girlfriend, shielded her from the bullets and saved her life.
For Caren Teves, it is her son's motives in the movie theatre that night - and not the killer's - that now matter most.
- The Washington Post's Katie Zezima contributed to this report.