CENTENNIAL, Colo. - The mother of Colorado theatre shooter James Holmes gave an impromptu public apology Tuesday, telling a judge during his sentencing hearing that her son feels remorse but his mental illness and medications made it hard for him to express it.

Arlene Holmes took the stand after more than 100 victims and survivors of the 2012 attack testified about the searing physical and emotional scars the shooting has left.

"We cannot feel the depths of your pain. We can only listen. ... And we pray for you," she said through tears. "We are very sorry this tragedy happened and sorry everyone has suffered so much."

Standing at a lectern with her husband, Robert, by her side, she told Judge Carlos A. Samour Jr. that she has been researching mental illness and ways to prevent mass violence.

"I am not proud I didn't know more about mental illness. We should have known our family history better and realized that the signs of mental illness can appear at a very early age," said Holmes, who previously testified that she didn't know her son suffered schizophrenia until after he killed 12 people and injured scores more in the attack.

Samour on Wednesday will formally sentence Holmes to life in prison without parole and up to 3,318 additional years on attempted murder convictions.

Jurors rejected Holmes' insanity plea, convicting him of murdering 12 people and trying to kill 70 others when he opened fire on a packed theatre in suburban Denver on July 20, 2012. Prosecutors have said the jury was divided on the sentence, with 11 favouring death and one favouring life without parole. Under Colorado law, jurors must be unanimous to impose the death penalty, so Holmes automatically got life.

On Tuesday, defence attorney Daniel King told the judge that Holmes would not be appealing his conviction, sparing victims the possibility of another emotionally wrenching trial.

More than 100 victims, free to vent their feelings during the three-day sentencing hearing, described haunting flashbacks, relentless survivor's guilt and physical pain that endures three years after the shooting.

Their statements, some quiet and reflective, some laced with anger and frustration, gave a glimpse into the far-reaching impact of the massacre on the community and beyond.

Marriages ended. Friendships shattered. Toni Billapando, who was shot and whose friend, Alex Sullivan, was killed, wondered about the effects of the shooting on her son, now 3. He was born just two weeks after the attack, as Billapando was recovering from her wounds. She was married at the time. Now, she's divorced.

"Imagine telling your child that monsters are real and not to be afraid of the dark when you're scared of the dark yourself," said another survivor, Stephanie Davies, who also has an 8-month-old son. She went to the theatre with her best friend, cradling her when she was shot. Now, the two no longer speak. "It was easier to heal ourselves in our own messed-up ways," Davies said in a statement read aloud by a prosecutor.

But the healing isn't over. Davies still sees the wide-eyed and bloodied faces of victims whose bodies she army-crawled over to escape the chaos. Billapando still struggles with knowing that she bought her friends the tickets. Others still refuse to step into a theatre.

A few called on Holmes, a once-promising neuroscience graduate student, to make something of his life in prison that could help prevent future attacks. Or, at least, to apologize.

John Gerhauser, whose best friend, Jonathan Blunk, was killed, told the judge: "If I were in charge, I'd say, this guy has to finish his Ph.D. and do something good for humanity or his death sentence will be reconsidered."