After Eric Harris and Dylan Klebold went into Columbine High School, 17 miles from Aurora, Colorado, on Adolf Hitler's birthday in 1999 and murdered 12 students and a teacher, the people who knew them could quickly point to a litany of disturbed writing and violent behaviour that led up to the attacks.
After Jared Loughner killed six people, and almost killed Congresswoman Gabrielle Giffords, in a shopping centre in Tucson, Arizona, last year, it was immediately possible to follow a trail of semi-political rantings he had left on YouTube.
And within minutes of Anders Breivik being identified as the man who massacred 69 young political volunteers on the Norwegian island of Utoya, it was clear he subscribed to a far-right militant ideology whose texts he had distributed online.
But for the distraught people of Aurora – and those across the US struggling to comprehend a mass shooting that killed and injured more people than any other in the country's blighted history – James "Jimmy" Eagan Holmes has left no such trail. Why did a high-achieving student, whom neighbours in his native San Diego remembered as a quiet but pleasant young man, who had drawn no attention to himself in his time as a doctoral student at Colorado University, drop out and begin planning a night of mayhem that killed 12 people and injured 58 others? The death toll, it was confirmed last night, included a six-year-old girl; among the critically injured was the child's mother.
It was tempting to recall the words of Alfred Pennyworth, Bruce Wayne's butler, in the last Batman movie: "Some men just want to watch the world burn." The words became an internet meme; they refer, of course, to Heath Ledger's maniacal Joker, on whom, it seems increasingly clear, Holmes modelled himself. If a photo taken from a profile on what is quaintly called an "adult dating site" really is Holmes, as police believe, he had recently dyed his hair bright orange. New York's police commissioner Ray Kelly revealed that friends in the Aurora force had told him that Holmes called himself the Joker when he was taken in, in the car park of the Century 16 cinema where he unleashed his massive cache of ammunition from behind his garb of riot gear and gas mask.
It was clear that Holmes's attack had been meticulously planned, perhaps for months, since there were reports he had approached a gun shop in California to ask about weapons to defend his home.
Officials report that he purchased four guns at "local metro gun shops" and bought more than 6,000 rounds of ammunition through online transactions in the past few weeks. He also bought a riot helmet, bullet-proof vest, gas mask and groin and throat protectors. Particularly deadly was a 100-round drum magazine, which meant he would be able to fire 50-60 rounds per minute.
Events were still unfolding last night, as bomb disposal teams worked to defuse what they believed was a booby trap at Holmes's student digs, four miles from the scene. Officials think the makeshift bomb was designed to lure emergency services to the outskirts of the suburb, so as to leave Holmes free to kill when he burst back into the cinema showing The Dark Knight Rises. For the fact that the police were not quick enough to respond to a noise complaint – he had left techno music booming in the flat – or that neighbours did not go in the apparently open door, the rest of the attendees of the movie showing will forever be grateful. Police were at the theatre 90 seconds after the first 911 call, police chief Daniel Oates said.
At Holmes's digs, Mr Oates told reporters: "I saw an awful lot of wire, jars full of ammunition and things that look like mortar rounds."
Police said last night they successfully disabled a second triggering device through a controlled detonation. But police were anxious to minimise destruction in the apartment: if there is any clue to Holmes' motive for his murderous spree, it will be there.
Mr Oates said: "We've become aware that he had a high volume of deliveries to both his work and home address. We think this explains how he got his hands on the magazine, ammunition. We also think it begins to explain how he got the materials he had in his apartment. What we're seeing here is evidence of some calculation and deliberation."
Those who knew Holmes described him as a shy, intelligent person from a well-to-do suburban San Diego neighbourhood. He played soccer at Westview High School and ran cross-country before going to college.
In the rundown neighbourhood of North Aurora, where Holmes had lived for little more than a year, and where the normal dangers are gun violence from drug dealers and pimps, there was shock, but few clues. Christopher Rodriquez, who lived in the flat downstairs, said he had barely ever seen Holmes.
Even those who had met him as recently as last Tuesday said they saw no sign of the planning that he had obviously already begun for his deadly attack. Jackie Mitchell said he recognised Holmes's photo on television as a guy he met at a bar on the Tuesday before the shootings. "We just talked about football. He had a backpack and geeky glasses and seemed like a real intelligent guy," he said.
Nineteen people remained in hospital yesterday, two in a critical condition, as a steady stream of information emerged. Relatives confirmed that Veronica Moser-Sullivan, six, was among the fatalities. Her mother Ashley, 25, was in hospital in a critical condition after being shot in the neck. She had not been told of her daughter's death. Grief counselling sessions for the public began at local high schools.
The hill opposite Century 16 is now littered with makeshift memorials to the dead. As people placed the signs and symbols of shrines, from candles to teddy bears and messages such as "Aurora is strong", the area was the focus of a vigil held by Christian churches the night after the massacre.
There, as church leaders loudly questioned the moral direction of the nation, on the sidelines some of the friends of victims were grieving more quietly. Candice Brown broke down when she received confirmation that her high school friend Aj Boik was among the dead. "It is indescribable he is gone," she said. He was with his girlfriend, Lasamoa Cross, whom he planned to marry. She survived; he did not.
MsBrown's friend, Michal Westrich, described Mr Boik as a pool ace, whose main concern was making everybody laugh. "If you looked anything but happy, he would come up to you and say, 'This ain't working, you got to smile.' "
On Twitter, Mr Boik's long stream of comic tweets – under the name AbsurdAsparagus – ended abruptly on Thursday night, but served as digital monument to his optimism and enthusiasm. In a message left after his death was confirmed, another friend, Jacob Hampton, wrote: "Find me a copy of Marvel vs Capcom 3 while you're up there. Rest in peace bro."
Last night Christian Bale, star of The Dark Knight Rises, released a statement on the massacre: "Words cannot express the horror that I feel. I cannot begin to truly understand the pain and grief of the victims and their loved ones, but my heart goes out to them." The identities of further victims were released by the Arapahoe County Coroner's Office last night. They include mother-of-two Rebecca Wingo, 32, whose father Steve Hernandez said: "We lost our daughter to a madman. My grief right now is inconsolable. I hear she died instantly, without pain, however the pain is unbearable." A friend left two teddy bears on behalf of her daughters and flowers for her parents close to the cinema.The coroner's list also included the names of cinema employee Alex Sullivan, 27, sailor John Larimer, 27, Micayala Medek, 23, Alex Tevez, 24, Jesse Childress, 29 and budding sports reporter Jessie Ghawi, 24. The coroner also confirmed the deaths of Jonathan Blunk, 26, who was shot as he tried to protect his girlfriend and 51-year-old Gordon Cowden, the oldest of the victims. A statement added that Matthew McQuinn, 27, had been "presumptively identified" as being among the dead.
Aurora massacre: The box-office fallout
The Dark Knight Rises had grabbed an impressive $31m (£19m) in ticket sales for its midnight launch. But this slid off the rails yesterday in the aftermath of the shootings in a cinema in Aurora, Colorado. Warner Bros said it would publish no further weekend box-office figures out of respect for the victims, and was considering removing images of gun violence in some trailers. Meanwhile security was stepped up in flagship cinemas, and remaining premieres were scaled back or cancelled. Following Paris, Tokyo and Mexico City also cancelled red-carpet premieres, with Warner Bros confirming that Christian Bale, Anne Hathaway and Morgan Freeman would no longer attend. In the UK, the Odeon chain searched filmgoers' bags in its flagship Leicester Square cinema, while the Vue chain said it was "liaising with the relevant authorities".
Stephen Foley and Kunal Dutta
John Larimer had joined the US Navy almost a year ago and was serving his first posting as a code-breaker and intelligence officer. The 27-year-old was remembered in his native town of Crystal Lake, Illinois, yesterday as one of five children, who had excelled at theatre at school. Marsha Pothoff, the principal of his high school, Crystal Lake South, told the Chicago Tribune: "He was a very nice young man, just a great kid to be around."
Matt McQuinn, 27, was attempting to provide cover for his girlfriend when he was shot by Holmes. Although she was hit by a bullet, Samantha Yowler is recovering in hospital, according to a family attorney. The couple had recently moved to Colorado from Ohio.
The 24-year-old aspiring sports broadcaster was the first victim publicly identified. Just weeks ago, the self-described "red-headed Texan spitfire" wrote of the fragility of life after seeing a shooting in a shopping mall where a man was killed. After she was shot, her friend Brent Lowak tried to give her first aid, but fled when the gunman began shooting at him. Jessica's brother, Jordan, said Lowak's actions were "nothing but heroic".
Family members called Micayla Medek, 23, a "free spirit" who was working in a sandwich shop while deciding what to do after college. Her family had to wait 19 hours before it was confirmed she was among the dead. Jenny Zacovich, the victim's aunt, told the LA Times how Cayla's father broke down when he heard the news. "He was absolutely hysterical, just sobbing, 'I want to get my baby and bring her home.'"
Aj Boik, 18, had graduated from the nearby Gateway High School. Always eager for a laugh, he tweeted under the handle @absurdasparagus. He was with his girlfriend, who survived the shooting. A friend tweeted, "Aj Boik was someone I'd take a bullet for because I know he will do the same for me."
Alex Teves, 24, was a therapist from Phoenix. Caitlin, a graduate student in Denver who had been at the premiere but escaped the shooting, announced Teves's death on Twitter. "Alex Teves was one of the best men I ever knew. The world isn't as good a place without him," she wrote.
Alex Sullivan, an avid comic book collector, nicknamed "Sully", died on his 27th birthday, and two days before his first wedding anniversary. He was an employee of the Century 16 cinema, watching The Dark Knight Rises on his night off.
A US Air Force spokesman confirmed the death of Staff Sgt Jesse Childress, who was an Air Force reservist on active duty with the 310 Force Support Squadron.
Profiles by Stephen Foley and Shannon Firth