(CNN) -- Aaron Alexis was under "the delusional belief that he was being controlled or influenced by extremely low frequency electromagnetic waves" before he embarked on a bloody shooting rampage at the Washington Navy Yard, an FBI official said Wednesday.
The 34-year-old contractor, who until a few years ago had served in the Navy, spelled out this belief -- with the words, "My ELF weapon" -- in the sawed-off Remington 870 shotgun he brought into the military facility's Building #197 on the morning of September 16.
"ELF" refers to low-frequency electromagnetic waves, a technology used for submarine communications that conspiracy theorists believe the government employs to monitor and manipulate unsuspecting citizens, the FBI said.
"Ultra low frequency attack is what I've been subject to for the last 3 months," read a message obtained by federal authorities from Alexis's thumb drives, phones and computers. "And to be perfectly honest, that is what has driven me to this."
"This" is the incident that left the Navy, Washington and the nation in shock. Alexis was quickly tabbed as the shooter. Still, big questions loomed as to why he did it, as well as whether he'd gotten help or told anyone about his plans.
Valerie Parlave, assistant director in charge of the FBI's Washington field office, stated unequivocally Wednesday "the investigation has determined that Alexis acted alone." Asked whether he talked to or e-mailed anyone ahead of the attack, Parlave responded simply: "No."
The public also got the first chance Wednesday to see Alexis in action that day, through surveillance video and photos.
They start with Alexis driving his rented blue Toyota Prius, with New York plates, around 7:53 a.m. into a parking garage, and then 15 minutes later carrying a backpack into Building #197. A still photo, taken afterward, shows that backpack hung on the inside wall of a fourth-floor bathroom stall -- where he'd put his shotgun together, authorities say.
The most chilling footage shows Alexis, dressed in a short-sleeve polo shirt and pants, prowling the building's halls and stairwells. Most of the time, his gun is cocked -- ready to fire.
Alexis went on to kill 12 people that day and wound four others.
The same shotgun on which he had written "My ELF" contained other etchings as well. The words: "End to the torment!" "Not what y'all say!" And "Better off this way!"
At some point during his rampage, Alexis picked up a Beretta handgun. He exchanged gunfire with law enforcement officers a few times during the more than hourlong ordeal, which ended at 9:25 a.m. with Alexis shot dead on the third floor.
Alexis had arrived in the capital area on August 25 for a contracting project, a few weeks after he told police in Newport, Rhode Island, that he'd heard "voices" emanating from the walls of hotels he'd been staying at.
Once in Washington, he stayed at two suburban hotels before settling into a Residence Inn in the city on September 7.
Two days later, Alexis began his contracting job based at the Navy Yard. On the first Saturday after that, he was at a Northern Virginia shooting range and gun store -- where he purchased the Remington 870.
FBI: Shooter did not target specific victims
A manager raised a performance-related issue with Alexis the previous Friday, but Parlave said that there is no indication this incident spurred any notable reaction. In fact, when he began shooting, it appears he wasn't picky about his victims.
"We have not determined there to be any previous relationship between Alexis and any of the victims," the FBI official said.
With his contractor status -- not to mention he was a former Navy reservist -- few questioned that Alexis legitimately gained access to the Navy Yard the morning of the fateful shooting.
Still, some have asked whether he should have had such access, given his mental health and criminal history.
The latter includes a 2004 arrest in Seattle, when he was accused of shooting the tires of a man's truck in an anger-fueled "blackout," according to a police report. Alexis' father told police his son had anger management problems associated with post-traumatic stress disorder, which he suffered after working "as an active participant in rescue attempts" during the September 11, 2001, terrorist attacks in New York.
Another arrest came in August 2008 in DeKalb County, Georgia, on a disorderly conduct charge. Two years later, Alexis was arrested in Fort Worth, Texas -- but never charged -- over an allegation that he fired a gun through the ceiling of his apartment.
That was the year, 2010, that the Navy moved to discharge Alexis due to what military officials described as a "pattern of misconduct" including insubordination, disorderly conduct, unauthorized absences from work and at least one instance of drunkenness. But due to a lack of evidence, he was instead given an honorable discharge.
Then there was his mental state, including last month's incident in Newport, Rhode Island. Alexis also sought help at two Veterans Affairs hospitals in and around Washington for sleep-related issues, according to law enforcement sources.
HP drops subcontractor Alexis worked for
Despite all this, Alexis still managed to find work at The Experts for about six months over the past year. That company said the last of two background checks it conducted in June on Alexis "revealed no issues other than one minor traffic violation."
Like the military, The Experts has come under criticism for not recognizing Alexis as a potential problem or threat.
A Hewlett-Packard spokesman told CNN on Wednesday the technology company -- citing its policy of adhering "to the highest standards of business practices and ethics" -- had dropped The Experts as a subcontractor.
"Based on what we now know about The Experts' conduct, including its failure to respond appropriately to Aaron Alexis' mental health issues and certain incidents recently reported in the press, HP has terminated its relationship with The Experts," the spokesman said.
The Experts responded, in a statement, saying it was "disappointed' by HP's decision and insisting it has met "all of our contractual obligations."
"The Experts had no greater insight into Alexis' mental health than HP, particularly given that an HP site manager closely supervised him, including during the events in Rhode Island," the company said.
FBI takes heat over video release
The FBI was criticized Wednesday night, both by Defense Department officials and victims' families following the release of the video.
"People in DoD are furious that FBI and other law enforcement officials unnecessarily put out this video footage today. It only adds to the pain of the families and brought life back to a killer," a senior Pentagon official told CNN. "Outside of law enforcement channels, the video was not viewed."
Another Defense Department official described the release as "gratuitous," indicating there was nothing to be gained from the release.
Law enforcement officials said they reached all but one of the victims' families to inform them beforehand.
And officials at the FBI argued that the video clarified various conflicting reports about the timeline of events, and would help to dispel various pieces of misinformation, including one incorrect story that the shooter was looking for specific victims.
Ronald Hosko, assistant director of the FBI's Criminal Division, said what was released was a far less painful version of the graphic video collected by authorities.
Navy leadership said that while officials knew the footage would be released, they were not told of the content. It is not known if anyone in Pentagon leadership had expressed their views with the FBI or Justice Department.
Sources also told CNN that the Naval Criminal Investigative Service, an arm of the Navy, was part of the decision regarding the distribution of the video, and saw the footage before it was released.
One victims' family reacted with equal dismay.
"I'm in total shock after reviewing the video. We have not even had time to begin to grieve," Theodore Hisey told CNN Wednesday evening, saying his family did not know the FBI was about to release the video.
His sister-in-law, Mary Francis Knight, was one of the 12 killed by Alexis.
CNN's Greg Seaby, Bill Mears and Leslie Bentz contributed to this report.