CENTENNIAL, Colo. -

Arapahoe County Sheriff Grayson Robinson said Monday that Arapahoe High School shooter Karl Pierson got into the school on the day of the shooting through a door that should have been locked, but wasn't.

The sheriff says it was not unusual for the door, which is located on the north side of the building, to be propped open because it is the only convenient way to get into that part of the building.

Sheriff Robinson says he doesn't believe the shooting would have been prevented had the door been locked. He said Pierson was determined to hurt people that day, and he would have found another way in.

The shooting happened at 12:33 p.m. on December 13, 2013. Sheriff Robinson says all indications are that Pierson planned and executed the shooting alone. The sheriff says Pierson was armed with a legally purchased gun, more than 125 rounds of ammunition, a machete and 3 Molotov cocktails. A total of 6 rounds were fired in the school and one of the Molotov cocktails was detonated.

The sheriff says Pierson purchased the gun used in the shooting on December 6. He continued buying ammo and other weapons, and making plans for the shooting until the morning of December 13. Robinson said Pierson went bowling on the morning of the shooting.

Investigators continue to believe that Pierson was targeting a librarian and debate team coach with whom he had a dispute. Robinson says a threat was made in relation to that dispute in September. The threat was reported to law enforcement. What happened after that is still under investigation.

Claire Davis, 17, was the sole victim in the shooting. She died on December 21 after spending eight days in the hospital.

Davis will be memorialized at the National Western Stock Show and Complex on January 1, 2014 at 1 p.m. Tickets are required. Click here for more information.