CENTENNIAL, Colorado  — A prosecutor declared Monday that two psychiatric exams found Colorado theater gunman James Holmes to be sane as he meticulously plotted a mass murder, considering a bomb or biological warfare before settling on a shooting so that he could inflict more “collateral damage.”

“Meticulous” was how Holmes twice described his plans during the exams, District Attorney George Brauchler said at the start of a long-awaited trial to determine if he’ll be executed, spend his life in prison, or be committed to an institution as criminally insane.

“Boom!” Brauchler said repeatedly as he showed pictures of the victims and the weapons that killed them on a screen, describing in detail how bullets pierced their organs and destroyed their limbs. “Boom!”

“I shot at people trying to get away. I shot at people trying to leave the theater … to make sure others wouldn’t follow,” Holmes later told a psychiatrist, the prosecutor said.

The former neuroscience student has pleaded not guilty by reason of insanity to murdering 12 people and wounding 70 at a midnight “Batman” premiere nearly three years ago.