Based on reading the various news interviews with victims and family members of the victims here is my guess as to what officially happened:
Adam Lanza did not work alone. He arrived at the school with another shooter. Adam was equipped with 2 of his mom's handguns and her shotgun in the trunk. The other shooter was equipped with an assault rifle.
They both potentially entered the school with the same mission: kill the children and some adults. This is done so that the public can be disarmed once news breaks of this story.
The 2 shooter theory makes sense as some witnesses claim the shooter was silent and others claim he was very vocal. However, both shooters had different missions. Adam's mission was to shoot the kids with the handguns and once he ran out of ammo, he was to go back to the car to get the shotgun out of the trunk and return to the school. The other shooter was to shoot the kids but also kill Adam and plant the rifle on him.
The killings were focused on 2 classrooms. One shooter could have been in each room. This would explain why there were rifle rounds found in the school as the second shooter had a rifle. Once Adam ran out of rounds in the handguns he left to get the shotgun and was followed out by the 2nd gunman. The 2nd gunman then shot at Adam outside the school before he could get the shotgun out. This would explain why one of the cars had bullet holes in it in the parking lot. Adam was unaware that the other gunman was to kill him.
Adam then potentially knew he couldn't just run away due to the open area because he would be an even easier target to hit. So possibly when the gunman went to reload, Adam decided to run back in the school to hide. The gunman chased him passed the gymnasium windows which would explain why 2 shadows were seen running by the gym. Once inside the gunman successfully killed Adam and planted the rifle on him then escaped.
This is only a theory, if parts don't make sense or you have a better scenario say it. I'm sure I might be missing some parts.
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