Aurora, Colorado (CNN) -- James Holmes, the suspect in Friday's shooting of dozens of people at a Colorado movie theater, was a "nice guy" who worked well with children, said a man who worked with him at a summer camp four years ago.
"I would never have thought he would do something like that," said Gabriel Menchaca, who worked with Holmes at Camp Max Straus in the summer of 2008.
"He was a little isolated, but he was, you know, a nice guy," Menchaca told CNN's Soledad O'Brien on Monday.
He couldn't believe it when he realized his former co-worker was the suspect in the shootings.
"Once I saw it was him, I was shocked. ... Something made him do a big change like that. I had no idea that later on he would be a murderer, a killer," Menchaca said.
Holmes is due in court Monday, where he is likely to face charges of first-degree murder in connection with the shootings that left 12 dead and 58 injured.
Holmes, a 24-year-old doctoral student in neuroscience, was arrested outside the Century 16 multiplex early Friday. He's accused of opening fire in the crowded theater during a midnight screening of the new Batman movie, "The Dark Knight Rises."
Holmes was not involved in any incidents at the summer camp, Menchaca said.
Camp Max Straus caters to needy children ages 7 to 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.
"It's a big responsibility to work in a camp like that, and he was living up to it. He was pretty good," said Menchaca, who did not stay in touch with Holmes after they worked together four years ago.
Holmes applied to join a gun range in June, but the range's operator said Sunday that he was disturbed by the "weird and bizarre" message on Holmes' answering machine.
"It was weird," said Glenn Rotkovich, managing partner of the Lead Valley Range in Byers, Colorado. "I didn't know what kind of message was left by this idiot. We need to know if he's an idiot before we let him have access to our range."
Rotkovich told CNN that Holmes sent in an online application on June 25. Rotkovich said he called the number Holmes left on the form, only to get a message he said was largely unintelligible.
He said the voice was "guttural, freakish, maybe drunk. Just weird and bizarre -- a deep, guttural, forced voice."
Rotkovich said Holmes didn't call him back. He left another message, and said he was again disturbed by the voice on the machine.
"It bothered me so much I told my staff, 'If this guy shows up, he gets nothing until I meet him,' " he said.
He said he tried calling Holmes a third time, again with no response, then quit. Holmes never called back, nor did he come to the range in Byers, about 40 miles east of Denver. But when news broke Friday of the bloodbath in a theater in the Denver suburb of Aurora, Rotkovich said one of his staffers remembered the name and saw that it matched reports of the suspect in the killings.
Pictures obtained of Holmes show a tall, bright-eyed, dark-haired young man, in contrast to the man a law enforcement official said had dyed his hair red and identified himself as "The Joker" after his arrest. And authorities recovered a Batman poster and mask from his apartment, a law enforcement source with direct knowledge of the investigation said Sunday.
It's an image difficult to reconcile for those who know him as a clean-cut, quiet graduate student.
He entered the University of California, Riverside, in 2006 on a scholarship 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.
Holmes' sole brush with the law in Colorado appears to have been a 2011 summons for speeding from Aurora police.
But there are indications that something may have been amiss in Holmes' life in recent months. He withdrew from the program in June, though "he gave no reason for his withdrawal from the graduate school," said Jacque Montgomery, a spokeswoman for the University of Colorado.
He received many deliveries over the past four months to 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 found in his booby-trapped apartment, Aurora Police Chief Paul Oates told reporters Saturday. Holmes purchased four weapons and more than 6,000 rounds of ammunition in recent months, Oates said.
Police haven't disclosed any motive for the attack or explained Holmes' appearance at the time of his arrest, citing an ongoing investigation.
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.
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 of the university.
CNN's Mariano Castillo, Susan Candiotti, Nick Valencia and Chelsea J. Carter contributed to this report.