CENTENNIAL, Colo. — James Holmes will spend the rest of his life in prison after at least one juror balked at the possibility of sentencing him to death for the massacre that claimed 12 lives and spawned a gut-wrenching four-month trial.

Because the 12 jurors failed to unanimously agree that Holmes should be executed, he will be sentenced to life in prison without parole for the chilling 2012 attack on a midnight screening of a Batman movie that also left 70 injured.

Nine jurors wanted to execute Holmes, but one was steadfastly opposed and two others wavering, a juror told reporters after the verdict was announced.

“Mental illness played into the decision more than anything else,” said the woman, who would not give her name. “All the jurors feel so much empathy for the victims. It’s a tragedy.”

The verdict shocked the courtroom. Holmes’ mother, Arlene, who had begged jurors to spare his life, leaned her head against her husband’s shoulder and began sobbing. In the back, Aurora police officers who responded to the bloody scene of Holmes’ attacks began crying.