James Holmes, 27, has admitted the killings, but pleaded not guilty on grounds of insanity.
Mr Holmes opened fire during the midnight premiere of a Batman film in Aurora, Colorado, in July 2012.
Officials say he was wearing body armour, threw smoke canisters and shot at people trying to escape.
If the jury finds him not guilty on grounds of insanity, he will spend the rest of his life in a mental institution for the criminally insane.
Otherwise he faces the death penalty.
During opening statements on Monday, District Attorney George Brauchler said both mental health evaluators agreed on Mr Holmes' mental state at the time of the attack.
"Both of them said the same thing - that the guy was sane when he tried to murder all those people in the theatre," he said.
Prosecutors have argued the attack was clearly premeditated, with Mr Holmes planning and amassing weapons for months, as well as elaborately booby-trapping his apartment to kill anyone who tried to enter.
His parents wrote a letter to the Denver Post in December calling for his life to the spared.
Robert and Arlene Holmes said their son was "not a monster", but suffers from severe mental illness.
The trial comes after two and a half years of complicated legal questions related to the death penalty and the insanity plea, and after nearly three months of jury selection.
The mental health evaluations could be a key factor. Judge Carlos Samour ordered a second mental health exam after prosecutors said the first one was biased. Defence lawyers objected to a second test.
At the time of the shooting, Mr Holmes was a graduate student in neuroscience at the University of Colorado in Denver, but was withdrawing from his studies after failing a key exam.
When he was arrested, Mr Holmes' hair had been dyed red in an apparent reference to the Joker, a Batman villain.
He appeared dazed in his first court appearance, and cameras were banned from further hearings.
A Colorado man who shot and killed 12 people in a cinema in 2012 was found sane by two experts immediately following the attack, prosecutors say.