(CNN) -- James E. Holmes is described by those who know him as a doctoral student who is clean-cut, quiet and responsible, an image difficult to reconcile with the same man who police allege opened fire in a crowded movie theater.
Days after the 24-year-old was arrested on suspicion of a mass shooting at the Century 16 multiplex in Aurora, Colorado, the portrait of Holmes that is emerging is as limited as it is confusing.
Pictures obtained of Holmes show a bright-eyed young man, who is tall with dark hair, which contrasts the description of the man by a law enforcement official who said he dyed his hair red and identified himself as "the Joker" to authorities after he was arrested early Friday morning for allegedly shooting people during a screening of the new Batman movie.
By all accounts, Holmes is a bright student. He entered the University of California, Riverside, in 2006 as a scholarship student and graduated with highest honors with a bachelor's degree in neuroscience in 2010.
"Academically, he was at the top of the top," Chancellor Timothy P. White said.
UC Riverside police have no record of any contact with Holmes, the university said.
Neither did police at the University of Colorado School of Medicine, where Holmes enrolled in 2011 as a doctoral candidate in its neuroscience program at the Anschutz Medical Campus in Aurora, university officials said in a statement released Sunday.
In fact, the sole contact authorities in Colorado appeared to have had with Holmes was a speeding summons in 2011, according to Aurora police.
A syllabus that lists Holmes as a student at the medical school shows that he may have taken a class in which he studied topics as diverse as substance abuse, schizophrenia, depression and other disorders.
According to the document, he was to have delivered a presentation in May about microRNA biomarkers.
Though there are indications that something may have been amiss in Holmes' life in recent months.
He withdrew from the program in June 2012, though "he gave no reason for his withdrawal from the graduate school," said Jacque Montgomery, spokeswoman for the University of Colorado.
It is not immediately clear if Holmes, who also worked in a paid position in the university's neuroscience research program, was still employed there after withdrawing from the program.
Holmes received a large volume of deliveries over the past four months to both his home and work addresses, which police believe begins to explain how he got his hands on some of the materials used in the rampage and the subsequent discovery of his booby-trapped apartment, Aurora Police Chief Paul Oates.
The police chief has declined to release details about a possible motive or Holmes' appearance at the time of his arrest, citing an ongoing investigation. But he did say Holmes purchased four weapons and more than 6,000 rounds of ammunition in recent months.
Police allege 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. Witnesses to the shooting say because the gunman wore a gas mask, they did not see his face.
Unlike most people his age, Holmes does not appear to have a social media footprint -- no Facebook, no Twitter and no Tumblr account, though authorities are investigating whether he posted a profile on 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.
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 on 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.
A neighbor who lives one floor below Holmes' third-floor apartment, Tori Lynn Everhart, described the apartments this way: "It's not like true ghetto. It's not the safest neighborhood, but it's definitely improving."
Apparently, Holmes told police during his arrest he had booby-trapped the third-floor apartment.
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 booby trapped with explosives.
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.
Bomb technicians worked Saturday to clear "all hazards" from the apartment, and began allowing some evacuated residents to return.
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."
In San Diego, the suspect's family issued a statement saying they were still trying to process the news.
"Our hearts go out to those who were involved in this tragedy and to the families and friends of those involved," the Holmes family said, without giving any information about him.
CNN's Tom Watkins, Susan Candiotti, Ross Levitt, Paul Vercammen, Casey Wian and Jim Spellman contributed to this report.