DENVER -- The parents of the gunman who killed 12 people at a Colorado movie theatre pleaded Friday for his life to be spared through a plea bargain --rekindling the long-running, emotional debate about whether the horrific details of the mass killing should be played out at his upcoming trial.

The statement released by Robert and Arlene Holmes emphasized a key legal issue in the tortured history of the case -- James Holmes' mental state when he staged the July 2012 attack, and whether he should die if convicted of the crime.

Holmes has pleaded not guilty by reason of insanity to the charges filed after the attack during a showing of a Batman movie in Aurora, outside Denver. Seventy people were also injured.

"He is a human being gripped by a severe mental illness," the parents wrote in just their second public comments since the attack. "We have always loved him, and we do not want him to be executed."

The statement also thrust the mass shooting into the spotlight once again at a time of heightened concerns over theatre safety. The hackers who prompted Sony to withhold the release of "The Interview" had threated violence against theatres.

Prosecutors previously rejected at least one proposed plea deal made by attorneys for Holmes, criticizing the lawyers for publicizing the offer and calling it a ploy meant to draw the public and the judge into what should be private plea negotiations.

They said the proposal could not be considered genuine because the defence repeatedly refused to give them information needed to evaluate it. No details were provided on the information but attorneys have argued in court previously about access to details concerning Holmes' mental health.

Jury selection is set to begin on Jan. 20 in the trial.

The comments by Holmes' parents incensed some survivors of the attack and relatives of those killed. They questioned the timing around the holidays and as 9,000 jury summonses were being delivered for the trial.

"To give a person who committed such a calculated, horrific crime a life sentence, to me that's like giving him a slap on the wrist," said Marcus Weaver, who was shot in the arm and whose friend, Rebecca Wingo, died in the attack.

Prosecutors, who are under a gag order, declined to comment, saying it would hurt the fairness and integrity of the trial.

His parents, who live in California, say they want to avoid a traumatic trial. One option would be a deal that calls for a guilty plea in exchange for a sentence of life in prison without parole, they said.

"If that happened, our son would be in prison the rest of his life, but no one would have to relive those horrible events at a trial the media has permission to televise," the parents wrote.

They say the best option would be sending their son to an institution for the mentally ill, which could happen if he is found not guilty by reason of insanity.

In addition, they say they have spent every moment since the shooting thinking about the victims and their families and friends.

"We are always praying for everyone in Aurora. We wish that July 20, 2012, never happened," they wrote.