The Navy Yard gunman did not enter Building 197 with a shotgun intent on targeting specific individuals. The FBI found evidence Aaron Alexis was motivated by what he described as an "ultra low frequency attack" he sustained for three months before the shooting, FBI officials said Wednesday.

Officials found a backpack that Alexis carried inside the Navy Yard that held the shotgun he used to shoot 16 people, killing 12. In it, investigators found a document that stated: "Ultra low frequency attack is what I've been subject to for the last 3 months, and to be perfectly honest that is what has driven me to this."

Alexis etched "End of the torment!" onto the barrel of the shotgun. He also etched "Not what yall say!" and "Better off this way!" on the right and left sides of the weapon. Alexis wrote "My ELF weapon!" on the left side of the shotgun.

Investigators said they believe that ELF stands for extremely low frequency electromagnetic waves. FBI officials explained in a release that ELF technology is a program for naval sub-tonal submarine communications.

This is the first time the government has released any type of motive to explain what caused Alexis to enter the Navy Yard on the morning of Sept. 16 and kill 12 people inside Building 197, the headquarters of Naval Sea Systems Command.

FBI investigators believed Alexis entered the Navy Yard prepared to die during the attack.

"He accepted death as the inevitable consequence of his actions," the FBI wrote in a release.

On Wednesday, the FBI also released video of Alexis walking the halls of Building 197 with a shotgun in his hand. The video shows him driving a Toyota Prius into the parking garage and Alexis walking into the building carrying a backpack. It does not show Alexis firing the shotgun.

In the video, Alexis is wearing a striped polo shirt, boots and dark slacks. He is wearing two identification badges. One is clipped to his shirt. Another is clipped to his belt.

Along with the Remington 870 shotgun, FBI officials said Alexis also used a Beretta handgun that he obtained during the course of the shooting rampage.

The FBI laid out the timeline from the time Alexis entered Building 197 at 8:08 a.m., until law enforcement shot and killed him on the third floor of the building at 9:25 a.m.

Reports emerged of possible multiple shooters on the morning of Sept. 16. This may have occurred because Alexis moved between the first, third and fourth floors throughout the hour-long attack giving the perception of multiple assailants.

Before shooting his first victim at 8:15 a.m., the FBI said Alexis entered the fourth floor men's bathroom at 8:09 a.m. carrying a backpack and a clipboard. He emerged from the bathroom carrying only the shotgun six minutes later.

Investigators found his backpack hanging from a stall in the bathroom. In it, officials found electronic media that offered clues to his motives and possible mental issues Alexis was battling, officials said. Investigators found more electronic media in the Residence Inn hotel room in Southwest D.C. that Alexis was staying the week before the shooting.

FBI officials did not describe what type of electronic media they found. Analysis of Alexis' belongings remains ongoing, investigators said.

Deputy Defense Secretary Ash Carter announced on Wednesday that the Pentagon would establish an independent panel to investigate the Navy Yard shooting and how Alexis received a security clearance despite the arrest records in his record.

The Navy and the Defense Department have launched similar investigations to explore the security and screening breakdowns ahead of the Sept. 16 shooting.