The man accused of shooting up a Colorado movie theater, dressed in full body armor with his hair dyed red in tribute to the comic book villain The Joker, remains an enigma: an apparently quiet, studious doctoral candidate with next to no cyber footprint in today's social- media-saturated culture.

James Holmes, 24, has no identifiable Facebook, Twitter or LinkedIn accounts and details provided by those who knew him are spare, making it difficult to paint a picture of a man accused of so horrific a crime, let along grasp his motive for allegedly doing so.

"People are using the word 'loner' to describe him, and that's not a fair representation," said a fellow doctoral student at the University of Colorado Anschutz Medical Campus who knew Holmes and agreed to be interviewed on condition of anonymity.

"He has friends. He's quiet and keeps a low profile, but we're all like that. We're PhD students. There's not a lot of time to do other stuff," the student said.

Holmes was arrested in a parking lot behind the cinema moments after the shooting spree and is being held in solitary confinement to protect him from other prisoners. He is due to make an initial court appearance on Monday.

Raised in San Diego, Holmes played on his high school soccer team and went on to become an honor student at the University of California, Riverside, in Southern California, where in 2010 he got a degree in neuroscience.

Last year, he began a doctoral program in neuroscience at the University of Colorado Denver, but he was in the process of withdrawing from the program.

University spokeswoman Jacque Montgomery said Holmes was one of six doctoral students in neuroscience at the school.

Reuters