The father of Colorado movie massacre gunman James Holmes has pleaded for his son's life.

Bob Holmes showed the jury photos of family holidays when he said James Holmes was "a really excellent kid" who became mentally ill and is not to blame for his July 2012 rampage.

The jury found Holmes guilty on all counts related to the attack, in which 12 were killed and 70 wounded. The panel of nine women and three men must now decide whether the former neuroscience graduate student will be executed or serve life with no parole.

The killer's father said he and his wife had no idea their son was suffering from mental illness before the massacre. They knew he had split up with his girlfriend and dropped out of graduate school.

"I assumed he might be depressed. That was our main concern," the shooter's father said, adding they had made plans to see their son.

Displaying photos of family gatherings, camping trips and home movies of the defendant as a child surrounded by relatives, defence attorney Tamara Brady asked Bob Holmes if he still loved his son, to which the gunman's father replied that he did.

He never wanted to be the centre of attention and he liked to keep how he was feeling to himself so that he didn't burden others. And he just always was looking out for my mom and wanted to make sure my mom was okay and taken care of
- Chris Holmes

He added that he first learned of the mass shooting via a phone call in the middle of the night from a journalist.

"He was clearly really messed up," Holmes Snr told the court. "He was able to talk to us, which was good, and he told us he loved us ... But I could see something was really wrong with him."

On 27 July, the gunman's younger sister broke down and sobbed as she became the first of his relatives to testify at the trial, telling jurors her brother's murders were completely out of character and that she still loves him.

"He never wanted to be the centre of attention and he liked to keep how he was feeling to himself so that he didn't burden others. And he just always was looking out for my mom and wanted to make sure my mom was okay and taken care of," said Chris Holmes.

Last week, the jury found the prosecution had proved "aggravating factors" which, the state argued, made Holmes' crimes especially heinous and deserving of execution.

Defence attorneys are now calling witnesses in the hope they can prove mitigating factors outweigh the aggravating ones.

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