Jurors weigh life or death for Colorado movie gunman Holmes
Accused Aurora theater gunman James Holmes listens during his arraignment in Centennial, Colorado March 12, 2013. REUTERS/R.J. Sangosti/Pool

Jurors in the Colorado cinema massacre trial were deliberating on Friday whether gunman James Holmes should be put to death for killing 12 people and wounding 70 three years ago inside a packed midnight screening of a Batman film.

The panel already convicted the 27-year-old former neuroscience graduate student last month on all charges from the July 2012 mass shooting in the Denver suburb of Aurora.

The nine women and three men must now decide if he will be executed, or serve life with no chance of parole. They must be unanimous that death is appropriate, otherwise Holmes will get an automatic life sentence.

The proceedings against him began in late April and reached penalty phase closing arguments on Thursday after 306 witnesses and the introduction of nearly 2,700 pieces of evidence.

In his speech to the jury, District Attorney George Brauchler said justice for Holmes meant execution for the "horror and evil" he wrought inside the crowded cinema.

During the trial, dozens of wounded survivors testified about how they had tried to hide from the gunman's hail of bullets, some of them steel-penetrating rounds, or stumbled over the bodies of loved ones as they tried to flee.

Defense lawyer Tamara Brady asked jurors whether they were ready to sign the death warrant of a mentally ill person and said they would have to live with the decision for the rest of their lives.

While the jury has already rejected Holmes' plea of insanity, Brady said all the doctors the panel had heard from in court had agreed that he was seriously mentally ill.

Holmes bought a ticket for the screening of "The Dark Knight Rises" before slipping out to his car behind the building and changing into what prosecutors called a "kill suit" of ballistic helmet, gas mask, and head to toe body armor.

He returned and threw a teargas canister into the theater, then opened fire with a semiautomatic rifle, pump action shotgun and pistol. Holmes has declined to testify in his own defense, or to make an allocution statement to the jury.