Authorities say the gunman entered Arapahoe High School at 12:33 p.m. He entered on the west side of the school.
A side door used by the Arapahoe High School gunman was supposed to be locked, but was left unlocked and ajar on the morning of the shooting. However, the Arapahoe County Sheriff said he does not believe the shooter would have been deterred by a locked door.
The Sheriff said he believes that Karl Pierson would have found another way into the school to carry out his "evil plan" saying that his goal was to inflict the as much harm as possible to as many people as he could.
The door, the Sheriff said, was habitually left unlocked as a matter of convenience for people entering the school.
The Sheriff said Pierson gave no indication that something was amiss the morning of the shooting. He said the suspect had a meal and even went bowling by himself that morning, and was also expected to be in class.
Although authorities believe Pierson acted alone, the D.A. said it's too early to say if other parties may be criminally liable for the tragedy.
The District Attorney said a dozen warrants have already been issued in the investigation and more are likely to follow. He emphasized the importance of being slow and methodical in an effort to conduct a thorough investigation, and said that warrants, affidavits and details about evidence will not be unsealed for quite some time.
A third party will be assigned to review the tragedy, and the response that followed. That review is likely to look at things such as a detailed description of the tragedy, a timeline of events, the Arapahoe County Sheriff's Office's initial response, the incident command system and the rolls and responsibilities within it, the response of additional law enforcement and emergency personnel, communications during the incident, public information, victim advocacy, how the crime scene was secured and investigated, and the impact of prior training, planning and preparations.
Speaking of the tragic events of December 13th, the Sheriff said, "It will forever change us as individuals, as public safety professionals, as educators, as parents and as a community, but it's up to us to decide if it changes us for the better or for the worse... While we may be hurting as a community, we will be better for this. We will absolutely be better for this. There is no question."