James Holmes, the 27-year-old responsible for killing 12 people and wounding 70 others in a Colorado movie theater shooting in 2012, was found guilty on multiple counts of first-degree murder and attempted murder on Thursday.

The jury of nine women and three men returned the verdict after less than two days of deliberation, marking the end of an emotionally wrenching trial in Centennial, Colorado, that lasted nearly three months. They judged Holmes guilty on all 165 charges, including possession or control of an explosive or incendiary device.

Following the guilty verdict, the trial will now move to a month-long sentencing phase in which the jury must decide if Holmes will receive the death penalty or life in prison without the possibility of parole. The sentencing phase could begin as early as Wednesday.

Holmes and his legal team never denied that he committed the crime, instead seeking to convince jurors that the onetime neuroscience graduate student was insane and therefore incapable of distinguishing right from wrong. During the trial, the defense said a total of 20 doctors had confirmed that Holmes suffers from schizophrenia .

In Colorado, prosecutors must prove a defendant's sanity. To show that Holmes was sane as he planned and carried out the July 20, 2012, attack, the prosecution called upon more than 250 witnesses, including everyone from survivors of the shooting to Holmes' former girlfriend and his former therapist. An FBI agent recalled how Holmes had booby-trapped his apartment with explosives to distract first responders from the theater attack, using jars of homemade napalm and other incendiary devices all meticulously wired together.

"That is logical. That is rational, and that is anything -- anything -- but psychotic," said District Attorney George Brauchler in closing statements on Tuesday, after reviewing the plans Holmes laid out in the months before the shooting. "That guy was sane beyond a reasonable doubt, and he needs to be held accountable for what he did."

The defense countered by painting a portrait of someone suffering from severe mental illness.

"The evidence is clear that he could not control his thoughts," defense attorney Dan King said in closing arguments. "He could not control his actions, and he could not control his perceptions. ... Only the mental illness caused this to happen, and nothing else."

King urged jurors to disentangle themselves from witnesses' emotional testimony, which he said might encourage a vengeful, retaliatory ruling, and to instead respect "the fortress of the law."

Tom Teves -- the father of shooting victim Alex Teves, who was one of three men in the theater to die shielding their girlfriends from the hail of bullets -- told Denver NBC affiliate 9News that he was shocked there could ever have been any question about Holmes sanity.

[The defense] is saying [Holmes] was very sick. Thats not the issue," Teves said before the jury reached its verdict. "The issue wasnt whether he was sick or not. Hes an evil thing, and we can argue that evil is sick. The issue is: Did he know it was wrong? He knew it was wrong. Its unequivocal. The facts state it, his actions state it.

Four different experts offered testimony on Holmes' sanity during the trial. They all agreed he was suffering from some sort of mental illness at the time of the shooting, but disagreed on the extent of the illness. While two court-appointed psychiatrists determined that Holmes was legally sane and could tell right from wrong, the defense called two doctors who argued the opposite.

While neither Holmes nor his parents testified during the trial, Robert and Arlene Holmes did sit behind their son . The two published a letter in December 2014 , pleading against the prosecution's pursuit of the death penalty.

"We do not know how many victims of the theater shooting would like to see our son killed," they wrote. "But we are aware of people's sentiments. We have read postings on the Internet that have likened him to a monster. He is not a monster. He is a human being gripped by a severe mental illness."

"We believe that the death penalty is morally wrong," they continued, "especially when the condemned is mentally ill."

The Associated Press notes that the insanity defense is successful only about a quarter of the time in felony cases in which it is raised nationwide, with that rate declining further in higher-profile trials.

This story has been updated with additional information on the details of the verdict.