The man suspected of shooting 12 people dead at a screening of the new Batman movie in Colorado was often seen carrying guns to and from his home and was described Friday as a "loner".
James Holmes, 24, was enrolled since June 2011 as a student in neuroscience at the University of Colorado, but was reportedly in the process of withdrawing from his PhD program.
In a photo released by the university, Holmes wore a burnt-orange T-shirt and sported a smile, but one neighbour at the apartment block where the suspect lived in Aurora recalled a subdued character.
"He was always wearing camouflage pants and a hat,'' said Gabriel Macias, a Mexican who works at a meat factory and recalled seeing Holmes carrying guns and weapon cases to and from his apartment.
"We did not know him well because he talked to nobody. He was always locked up behind his door,'' said Macias.
A second photo, from Holmes's San Diego high school yearbook, showed him in a dark suit, this time flashing a toothy smile. He graduated from Westview High School in San Diego in 2006.
Melvin Evans, a security guard from a nearby building said he occasionally saw Holmes at a local bar.
"He was always by himself. Looked like a nice guy,'' said Evans, 33. "I go to the bar every Sunday for Karaoke. I've seen him there every now and then. We would talk sometimes, the weather, you know.''
The FBI described Holmes as a white male, 6ft 3in tall, born on December 13, 1987, with no significant criminal record and no links to terrorism.
Residents at the apartment block were removed in the wake of the shooting after it appeared that Holmes's home had been booby-trapped.
Police in Aurora, Colorado, told reporters records showed that the shooting suspect's only infringement in the city was a speeding ticket in 2011.
In an apartment rental application he submitted in early 2011, Holmes described himself as a "quiet and easy-going" student, the Denver Post said on its website.
"No one knew him. No one,'' it quoted a pharmacy student living in the same building, reserved for medical students, faculty and staff in Aurora, as saying.
The student, who only gave his name as Ben, said Holmes kept to himself and would not say hello or acknowledge other people in the hallway.
Ben also revealed he had called police shortly after midnight - coincidentally around the time of the massacre at a Batman premiere - to report a song blaring from inside Holmes' apartment.
Ben could not make out the song, but it appeared to be playing on repeat.
Tom Mai, a next-door neighbour of the Holmes family in San Diego, remembered Holmes as a shy teenager who did not play or socialize with other youngsters in the neighborhood, the San Diego Union-Tribune newspaper said.
"He said that he last saw him two years ago when he came home during summer recess from college,'' it said on its website. "The family, he said, was nice and involved with a Presbyterian church.''
Holmes graduated from the University of California in Riverside in 2010 with a degree in neuroscience, said its director of communications Kris Lovekin, who added she could not release any further information about him.
Mashable.com, which monitors social media, said Holmes was "an online ghost'' with no apparent presence on sites like Facebook or Twitter, although people sharing Holmes' name found themselves suddenly bombarded with attention.
In a statement, Holmes' family said they were cooperating with investigators.
"We are still trying to process this information and we appreciate that people will respect our privacy,'' the family said, after Holmes' mother Arlen confirmed her son was the suspected shooter.