(CBS News) Sources say investigators have found key evidence in dumpsters and trash bins outside of James Holmes' apartment, including a shipping label from an internet ammunition store called BulkAmmo.com.
Investigators also tell CBS News they have recovered a surveillance video of James Holmes picking up approximately 150 pounds of ammunition at a Federal Express outlet in Colorado.
They've interviewed a UPS driver who says Holmes had 90 packages delivered to his workplace on the University of Colorado medical campus.
And sources say over the past several months, Holmes spent about $15,000 as he was putting together his deadly arsenal -- guns, chemicals, explosives, and ammunition.
It all adds up to a methodically planned attack, said Aurora police chief Dan Oates.
"What we're seeing here is evidence I think of some calculation and deliberation," said Oates.
Investigators suspect Holmes accelerated his planning in late May, when he purchased the first of the four guns found at the shooting scene. Over the Internet, police say, Holmes bought a staggering amount of ammunition.
"Through the Internet he purchased over 6,000 rounds of ammunition, more than 3,000 rounds of. 223 ammunition for the assault rifle, 3,000 rounds of .40 caliber ammunition for the two Glocks in his possession and 300 rounds for the 12 gauge shotgun."
Holmes did most his shopping on Internet sites. And he was clearly in a hurry.
A receipt from TacticalGear.com shows on July 2 -- two-and-a-half weeks before the shootings -- Holmes purchased a bulletproof vest and other equipment. He paid extra to have it shipped by 2nd-day air.
But investigators still don't know what moved a quiet neuroscience grad student to commit mass murder. Police are now interviewing former classmates, neighbors, and co-workers looking for any explanations.
Police hope more definitive clues will be found in Holmes' apartment. Investigators will be looking for computer files, emails, and any communication records which point to motive.