No video of the actual shooting is posted, but Alexis can be seen jogging through the building - often in a crouched position - armed with a shotgun.
Alexis killed 12 people in the attack. Investigators say the 34 year old had a history of mental problems.
Valerie Parlave, head of the FBI's field office in Washington, said Wednesday during a news conference that investigators are continuing to explore his background and motivations.
Parlave says Alexis held a delusional belief that he was being controlled by extremely low frequency waves, or ELF waves. Alexis had written "my ELF weapon" on a shotgun he used in the rampage.
Alexis, a former Navy reservist and IT contractor, was killed in a shootout with police.