The 24-year-old former student who was arrested over the shooting at a Denver cinema was refused gun club membership after the owner heard his "bizarre" answerphone message.
James Eagan Holmes, who reportedly referred to himself as "The Joker" on the evening on the attack that left 12 people dead, applied to join the Lead Valley Range in Byers, Colorado last month.
Owner Glenn Rotkovich said when he called Holmes to invite him to a mandatory orientation session he heard a vociemail message that was "bizarre - guttural, freakish at best".
He left two other messages but eventually told his staff to watch out for Holmes at the July 1 orientation and not to accept him into the club, he said.
Rotkovich added: "Looking back, and if I'd seen the movies, maybe I'd say it was like the Joker."
Holmes, who had recently died his hair red, had just dropped out of a doctoral degree programmme in neuroscience at the University of Colorado's Anschutz Medical School, a few blocks from his apartment.
However, it is not clear why he quit his course and friends and fellow students painted a picture of a quiet, introverted academic.
A fellow student, who wished to remain anonymous, told Reuters that Holmes had complained of academic pressures but that was not unusual among people on the course.
"We all think the same thing as everyone - that he was pushed past his breaking point. But I personally don't know what that breaking point was," the student said.
"He was very quiet, that's true. And he did not seem to me to be the kind of person who would take matters into his own hands like this."
Billy Kromka, a research assistant in a neuroscience lab where Holmes was a student for several months last year, said: "He basically was socially awkward but not to the degree that would warrant suspicion of mass murder or any atrocity of this magnitude.
He said Holmes was not physically threatening, never talked politics or got animated about other issues, describing conversation with him as "typically pretty banal".
Neighbours in the apartment block, where police found his home rigged with booby traps and tripwires, said he barely acknowledged others and kept himself to himself.
Raised in San Diego, Holmes played in his high school football team and went on to become an honour student at the University of California, Riverside, where he gained a degree in neuroscience in 2010.
Ritchie Duong, who knew Holmes at high school and went to Riverside with him, told the Los Angeles Times he last saw the suspected gunman in December and he seemed "fine".
"I had one college class with him and he didn't even have to take notes or anything. He would just show up to class, sit there, and around test time he would always get an 'A'," he said.
His family still lives in San Diego and have said: "Our hearts go out to those who were involved in this tragedy and to the families and friends of those involved."
The family pastor Jerald Borgie recalled that Holmes was a shy boy who rarely initiated conversation himself.
"He had some goals. He wanted to succeed, he wanted to go out, and he wanted to be the best," he said. "He took pride in his academic abilities. A good student. He didn't brag about it."
