Prosecutors in the US state of Colorado say they will seek the death penalty for the man accused of killing 12 people in a cinema last July.
James Holmes, 25, is charged with multiple counts of murder and attempted murder in one of the worst mass shootings in United States history.
Dozens were wounded in the attack at a midnight showing of a Batman film.
On Friday, prosecutors rejected an offer from Mr Holmes to plead guilty in order to avoid execution, the BBC reports.
Mr Holmes' defence lawyers were expected to argue he is not guilty because he was legally insane at the time of the 20 July shooting.
But investigators say the former neuroscience graduate student had stockpiled weapons and ammunition ahead of the attacks.