Holmes was dressed in black, wearing a ballistic helmet, a tactical ballistic vest, protective leggings, a throat and groin protector, and a gas mask during the attack, police said, and witnesses say they did not see his face because of the mask.
Unlike most people his age, Holmes does not appear to have a social media footprint -- no accounts with Facebook, Twitter or Tumblr, though authorities are investigating whether he posted a profile on the sex website Adult Friend Finder.
The profile contains a picture of a man with fiery red hair, a law enforcement source said. Police believe it may be a picture of Holmes, said the source, who was not authorized to speak to the media. The profile said it was created by a 24-year-old man from Aurora and has since been taken down.
Adult Friend Finder said Sunday that it "values the privacy of its members" and would not confirm the identity of any users. The site will respond to police inquiries "as may be required by law," company spokesman Lindsay Trivento said in a written statement.
It's a profile that contrasts with news that Holmes worked as a counselor at a summer camp for needy children in 2008.
Camp Max Straus caters to needy children ages 7-14, and is run by Jewish Big Brothers Big Sisters of Los Angeles, which confirmed his employment but would not offer further details or comment.
The man who grew up in the upscale northwest San Diego neighborhood of Rancho Peñasquitos was renting an apartment in Aurora, according to police and realty records. Tom Mai, a neighbor of the Holmes family in San Diego, described Holmes as "clean-cut, quiet, responsible."
At the time of his arrest, Holmes was living in a small, three-story brick building on Paris Street in Aurora, in Apartment 10, within walking distance to the university.
"Neighbors report he lived alone and he kept to himself," Oates said.
Jackie Mitchell, who lives close to Holmes, had a beer with him on Tuesday.
Mitchell was stunned at news of Holmes' alleged involvement in the attack.
"You would never guess he was a violent guy," Mitchell said, describing Holmes as "nerdish" and "a book-smart type guy."
Holmes told police during his arrest he had booby-trapped the third-floor apartment, prompting investigators to evacuate the building and four others nearby and Friday and Saturday trying to disarm and get rid of a variety of explosive devices left behind.
Kaitlyn Fonzi, a 20-year-old grad student and neighbor, said she heard techno music blasting from his apartment around the time of the shooting and had nearly opened Holmes' unlocked apartment door to complain -- unaware that the unit was wired to explode.
A timer had turned the music on so that it would blare in his apartment after he left for the Aurora multiplex, according to a law enforcement source, who also spoke on condition of anonymity.