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. "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.