CENTENNIAL, Colo. -- The father of Colorado theatre shooter James Holmes testified Wednesday that his son once came home from university looking haggard and making odd facial expressions, but the parents didn't suspect at the time that he was descending into mental illness.

Jurors are considering whether Holmes should serve life in prison without parole or be executed for his July 2012 attack on the audience of a Colorado movie theatre, which killed 12 people and injured 70. Defence attorneys say he should get a life term because he is severely mentally ill.

Robert Holmes said in court Wednesday that his son stopped returning his and his wife's calls months before the attack and his psychiatrist called them in June, saying he was dropping out of his prestigious neuroscience program.

"We didn't know he was seeing a psychiatrist," Robert Holmes said. He and his wife thought then their son was depressed or suffering Asperger's syndrome, but he said the doctor would not return their calls seeking more information.

Holmes did send his parents sporadic and terse emails that gave no hints of trouble, and their concerns were eased again when they finally reached him by phone on July 4, 2012, just two weeks before the shooting. Their son was more talkative than usual and "he didn't give any indication he was homicidal or depressed, at least not to us," Robert Holmes said.

"He was not a violent person. At least not until the event," Robert Holmes said. "The event" is a phrase he used several times to refer to his son's attack.

Defence attorneys will also call James Holmes' mother, Arlene Holmes, to the stand as they prepare to rest their portion of the sentencing phase, which has included several dozen family friends, teachers and former neighbours who said the Holmes they knew was shy, mild-mannered and polite-- not the kind of young man who would gun down innocent strangers.

Death sentences must be unanimous, and the judge has explained to jurors that their decision will be highly personal. While jurors have already found Holmes was legally sane at the time of the attack, his defence is hoping at least one juror will agree that his mental illness and family ties reduce his moral culpability so much that he deserves the mercy of a life sentence instead.

Jurors have been shown pictures and home-movies from Holmes' unremarkable childhood: playing soccer, graduating high school, smiling at the dinner table, jumping in the surf near their quiet California neighbourhood.

Meanwhile, District Attorney George Brauchler sought to focus on what they didn't know or didn't tell jurors: that James Holmes' mother took him to a counsellor when he was just 8 because he was throwing things and acting out, and that once he was in college, he lost touch with his younger sister, and never inquired about her well-being.