The video begins innocuously enough. A blue Toyota Prius rental car pulls into the parking garage of the Washington navy yard at 7.53am on 16 September, then a man wearing a blue striped shirt with a backpack over his shoulder enters building number 197.

Suddenly, the mundane events of an ordinary Monday morning turn to images of horror. In the next clip the man is captured walking down a corridor, apparently calmly, with a Remington 870 shotgun in his hands. He pokes the barrel of the gun, sawed off for maximum impact, into the offices he passes at the start of the one-hour rampage that would end with the killing of 12 random victims, four wounded survivors and his own death.

The man is Aaron Alexis, 34, the shooter in the Washington navy yard tragedy who was trapped in the delusion, FBI agents said, that he was controlled by Extremely Low Frequency (ELF) electromagnetic waves. Stills images released by the bureau give clues to that delusion, and that he was prepared to die in carrying out his actions.

Etched into the left side of the receiver of the gun was the expression: "My ELF weapon!" Alongside it was the phrase: "Better off this way!" On the other side of the receiver the sentence had been scratched: "Not what y'all say!" And on the barrel itself was written: "End to the torment!"

The FBI released the video footage along with the still pictures of Alexis's movements and the shotgun and a new timeline of the carnage in the navy yard, It said the intention was to provide "greater understanding of the recent movements and activities of the deceased shooter". The video also provides a shockingly intimate window into the navy yard disaster as it unfolded.

In the final clip of the released footage, Alexis is seen running down a stairwell and entering a lower corridor, more urgently now. He enters the hallway, takes cover behind a wall, as a stream of people can be seen running at the end of the corridor; then he begins running towards them.

As the investigation into the navy yard rampage has progressed, the extent of Alexis's mental health problems have become apparent as well as numerous holes in safety procedures through which he fell. Earlier this week it was revealed that the federal contractors who carried out a background check on him in 2007 were aware that he lied on his application form and that he had covered up a previous arrest and charge for a firearms offence; yet Alexis was still granted secret security clearance in the navy.

The new details released by the FBI show Alexis's movements leading up to and during the rampage. Fifteen minutes after he entered the parking garage, he entered building 197 and went up the elevator to the fourth floor. He visited the men's bathroom, leaving his backpack and clipboard in a cubicle there.

At about 8.15am Alexis crossed the hallway into the 4 West area of building 197. He carried the Remington shotgun which he had bought just two days earlier at a gun shop in northern Virginia. That same day he also bought a hacksaw for using in sawing-off the barrel and stock.

His first victim was shot at 8.16am in the 4 West area, and a minute later calls started flooding in to 911 emergency lines. Over the next hour he moved to the first floor and the third floor, where he was shot and killed by officers at 9.25am.

FBI agents have carried out forensic examinations of the contents of the backpack, the Toyota Prius, and the Residence Inn in south-west Washington where he stayed from 7 September. In the course of their searches they discovered a roll of purple duct tape in the backpack which Alexis used to wrap the end of the sawed-off stock. They also found several documents stored on items of electronic media that included a phone, laptop and thumb drives.

Those documents have given investigators a sense of his state of mind at the time of the shootings, yielding clues to his intentions. In one of the documents, Alexis had written: "Ultra-low frequency attack is what I've been subject to for the last three months, and to be perfectly honest that is what has driven me to this."

Valerie Parlave, assistant director in charge of the FBI's Washington field office, said that the electronic documents indicated that "Alexis was prepared to die during the attack and accepted death as an inevitable consequence of his actions." She added that there was no evidence that he had targeted specific individuals – his victims were randomly selected.

The electronic communications did not indicate, Parlave said, that Alexis had been in touch with anyone before the rampage to alert them to what was to come.

The FBI said that it was aware that ELF electromagnetic waves were the subject of conspiracy theories. In a statement on its website, the bureau said: "ELF technology was a legitimate program for naval sub-tonal submarine communications; however, conspiracy theories exist which misinterpret its application as the weaponization of remote neural frequencies for government monitoring and manipulation of unsuspecting citizens."