Arapahoe High School shooter Karl Pierson was carrying more than a shotgun and multiple rounds of ammunition when he opened fire at the Denver school on Friday.
The 18-year-old was also armed with a machete and three Molotov cocktails, leading investigators to believe he intended to cause mass carnage.
Just before 12:33 p.m. on Friday, Pierson parked his car and walked into a door at the school adjacent to the library. He was wearing a bandolier of shotgun shells and carrying a pump-action shotgun.
"He took no effort to conceal the fact that he was armed," said Arapahoe County Sheriff Grayson Robinson.
Pierson fired point blank at student Claire Davis, who is now in critical condition.
"There was no time for the victim to run from the shooter," Robinson said Saturday.
He then fired another shot down a hallway and entered the library.
He fired again and then ignited one of the Molotov cocktails, setting at least three bookshelves on fire.
He fired another shot, retreated to a back corner of the library, and “there he took his own life.”
The attack lasted only 80 seconds, authorities said. Robinson said the incident could have been much worse.
"The individual also had multiple rounds of shotgun ammunition strapped across his body, and he was also armed with a machete," Robinson said. "It is our strong belief that (the gunman) came to this school with that weapon and multiple rounds, and his intention was to utilize those multiple rounds to cause harm to a large number of individuals."
The high school is just a few miles away from the Aurora cinema where 12 people were shot dead and 70 others were injured during a screening of Batman.