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.