Police thought Holmes was officer at first

CENTENNIAL, Colo. (AP) - A police officer who arrested James Holmes after the Colorado movie theater massacre thought he was a fellow officer at first because of the man was wearing body armor.

The officer then realized Holmes wasn't since he wasn't running toward the theater.

The officer and a colleague testified at a preliminary hearing Monday that Holmes seemed detached and volunteered that his apartment had been booby trapped.

Prosecutors are laying out their case against the former neuroscience graduate student at the hearing.

Investigators say Holmes tossed two gas canisters and then opened fire during the midnight showing of the Batman movie "The Dark Knight Rises" on July 20, killing 12 people and wounding dozens.

The hearing is expected to last all week.