LOS ANGELES (AFP) - "Batman" theatre gunman James Holmes will be back in court on Wednesday to face sentencing, a week after his conviction over the 2012 massacre that left 12 dead and 70 more injured in Colorado.

Prosecutors are pressing for the death penalty for the 27-year-old - who pleaded not guilty by reason of insanity - in the sentencing phase of the trial, which climaxed days before the third anniversary on Monday of the massacre.

Judge Carlos Samour has said he hopes the punishment phase of the trial - which could also jail him for life without parole - will end sometime in August.

Holmes was found guilty of opening fire during a packed screening of "The Dark Knight Rises" in Aurora, Colorado, in a meticulously-planned attack wearing body armour and using tear gas to prevent victims escaping.

The sentencing part of the trial comprises three phases, which the jury takes one by one - progressing to the next phase depending on their finding in the previous one.

In the first phase, the prosecution must prove beyond a reasonable doubt that the crimes included at least one aggravating factor. These include killing a child under 12, and acting in a "heinous, cruel, or depraved manner."

If that is proven, the defence gets to present its case for mitigating factors, which could include that Holmes's ability to tell right from wrong was significantly impaired, or that he was under particular duress.

Only if they reject the defence case do jurors move on to the third and final phase, to decide whether the prosecution has proved beyond a reasonable doubt that the death penalty is the appropriate sentence.

- Killer in black -

Wrapping up his case last week, prosecutor George Brauchler ran through a blow-by-blow account of the July 20, 2012 massacre, which stunned America and reignited the country's perennial debate about gun control.

Referring to the 400 people in the theatre, he said: "They came in hoping to see a story of a hero dressed in black, someone who would fight insurmountable odds in the name of justice and trying to protect others.

"Instead a different figure appeared by the screen dressed all in black. And he came there with one thing in his heart and in his mind - and that was mass murder," the prosecutor said.

But Holmes's defence lawyer Dan King insisted his client was insane, saying: "The fact of the matter is that when Mr Holmes stepped into that theatre... he had lost touch with reality.

"You cannot divorce the mental illness from this case, or from Mr Holmes. The mental illness caused this to happen. Only the mental illness caused this to happen, and nothing else," King said.

The 49-day-long trial included weeks of grisly evidence and testimony from more than 250 witnesses. The jury also was shown hours of videotaped interviews of Holmes by psychiatrists for the state.

Both they and two defence psychiatrists agreed the defendant struggled with mental illness.

The state's psychiatrists, however, maintained that Holmes was sane when he carried out the rampage.

If Holmes had been found not guilty by reason of insanity, he would have been confined to a state mental hospital.