James Holmes' lawyers say police coerced and misled him into discussing the bombs in his apartment after the Colorado theater shootings.
In a pretrial hearing Monday, defense lawyers said anything Holmes told officers shouldn't be used against him, arguing that police violated his rights.
Holmes pleaded not guilty by reason of insanity to charges of killing 12 and injuring 70 in the 2012 attack in suburban Denver.
Prosecutors and the defense are arguing over evidence that could be used to support or undermine his insanity claim, such as the bombs that police have said were intricately planned to divert officers from the theater.
Prosecutors argue police did nothing wrong in questioning Holmes.