(CNN) -- The gunman who opened fire at a Colorado high school this month came into the school through a door that should have been locked, a sheriff said Monday.
"We know the doorway on the north side that the murderer entered is supposed to be locked. Unfortunately, it rarely is, because it is more convenient for people to come and go from that area. The door that he entered was not secured on December 13," Arapahoe County Sheriff Grayson Robinson told reporters at a press conference. "He entered without any obstruction."
The gunman legally bought a shotgun from a local retailer after passing a background check on December 6 -- a week before the shooting at Arapahoe High School -- and spent a week stocking up on ammunition, Robinson said.
The shooter was still trying to buy more shells less than an hour before he entered through the unlocked side door carrying the shotgun, a machete, three Molotov cocktails and 125 rounds, the sheriff said.
Authorities have identified the shooter as 18-year-old Karl Pierson, who was a student at the high school. But at Monday's press conference, Robinson didn't mention his name, describing him repeatedly as a murderer who was "bent on evil" and "causing the largest amount of harm to the largest amount of people that he could."
Investigators believe Pierson plotted alone to carry out the deadly attack.
"We know that on the morning of December 13 the murderer conducted his business as he normally would, gave no indication that anything was amiss, took time to have a meal and actually took time to go bowling by himself alone," Robinson said.
Pierson fatally wounded 17-year-old Claire Davis in the attack before turning the gun on himself.
He also set off a Molotov cocktail in the library, creating a large amount of smoke and a fire that burned four bookshelves.
The attack took less than one minute and 20 seconds, Robinson said, "from the time that the murderer entered the school to the time his body was lying on the floor."