Aaron Alexis used a remote control to adjust the distance between his rifle and the cardboard cutouts – human silhouettes suspended from an electric rail on the ceiling at Sharpshooters, a gun store in an industrial park in northern Virginia.  It was Saturday, and the 34-year-old former navy reservist, wearing ear muffs, was standing upright in one of 16 cubicles at the indoor shooting range, at the back of the store. He was using a hired rifle for target practice.  He casually enquired about a shotgun displayed behind a glass cabinet on the wall, but opted instead for a pump-action Remington 870 shotgun and 24 shells, purchased for $419.  Less than 48 hours later, and just 17 miles from the gun shop, Alexis stood on the fourth floor of a secure navy administration building in Washington DC. Chunks had been sawed off the barrel and butt of the Remington gun, which he had been etched with the words "better off this way". The ammunition was in his front pocket.  Minutes later, a message crackled over DC emergency service radio:  "We got a report on the fourth floor. A male with a shotgun. Multiple shots fired. Four people down."  The story of what happened inside building 197 of the navy yard at first emerged only in fragments, as employees began sending messages to loved ones or finding ways to escape the base.  Tim Jirus, 48-year-old navy commander, saw a maintenance worker shot in the head right in front of him. He fled, clambering over a 20ft spiked fence. "We were just standing there, maybe three feet away, having a conversation, and then we heard two more gunshots, and he went down," he told TV cameras outside. "And that's when I ran."  By that point, 12 civilian employees had been murdered by Alexis, a disturbed information technology contractor who stalked the office block in search of people to kill, before he was shot by police. His victims were aged between 46 and 73 and were civilian staff working for the navy – either directly or for contractors.  Five days into the FBI investigation, law enforcement officials have told the Guardian there is now a growing focus on the mental health issues from which Alexis had apparently been suffering. Forensic and ballistic experts will finish their work in building 197 on Friday; they are treating the location of each fatal shooting as a separate crime scene, carefully examining the trajectory of bullets in order to piece together what happened.  The mass shooting at the headquarters of the naval sea systems command – a short distance from Capitol Hill – was unprecedented for DC, a city of fortified government buildings patrolled by armed guards. The navy yard is the oldest military station in the United States, with on-site museums and impressive views over the Anacostia river. About 18,000 staff work at the complex, about 3,000 in building 197, most of whom were either on or near the sprawling compound.  It began with two rapid bursts of fire.  "We panicked first, and we didn't know which door to go out of," said Patricia Ward, a 53-year-old logistics manager who was buying her breakfast when the shooting started. "We didn't scream, because we were in shock, running."  Outside, a female security guard was urging people to run. She had her gun drawn, and said she was going in to confront the rogue shooter. Gopi Upreti, 60, a computer specialist with Booz Allen, was walking past the other side of the building when he saw two terrified men sprinting out. "They were shouting 'gunfire on the fourth floor,'" he said.  The FBI now has a provisional idea of the route Alexis took. He began on the fourth floor then moved to the third, before entering the atrium, where he shot a security guard and took a handgun from the guard's holster.  He moved quietly, searching for victims in hallways or in the labyrinth of cubicles in the open-plan office space. Many office workers were hidden under their desks or cowering in cupboards and were lucky to escape. John Weaver, who knew six of the victims, said Alexis only overlooked him because he was focusing on shooting a female co-worker nearby.  Fortunately, the bullet only scraped the back of her head, but blew off most of her finger. "The force of the blast was so strong it drove her into the ground, and now she has a black eye and bits of her scalp are scattered all over her cubicle," Weaver told CNN.  The first police armed response unit entered the building within seven minutes of the initial 911 call. Soon, around 20 armed officers were in or near the building, marching up and down corridors in pursuit of the gunman.  But Alexis was moving quickly. He was reportedly seen in the lobby, by a staircase, and then behind a pillar on the first floor. There were repeated firefights with officers who caught glimpses of him but for a while it seemed the lone gunman had the upper hand. He repeatedly shot one DC police officer in the legs, although the officer survived.  On the third floor, navy captain Christopher Mercer slammed his office door closed when he saw Alexis coming, and then saw bullets puncturing the wooden door. "He set up camp right in front of my office. He kept reloading and firing at cubicles," Mercer told the Washington Post.  "Later, when he came back, I could see his shadow through the glass pane in my door."  That incident was followed by a pause in gunfire that lasted anywhere between 10 and 25 minutes. It was long enough for people still inside building 197 to wonder if it was now safe to come out.  Mercer told the Post he only realised Alexis was still at large when he heard his footsteps, along with the clicking sound of a shotgun being reloaded. He peered out to see the tall, lean figure of Alexis, who was moving furniture around. "I couldn't believe he came back – to the same spot," he said. Crouching under his desk, Mercer used his BlackBerry to email commanders details of precisely where on the floor Alexis was barricading himself in.  It ended around 9am, much as it had begun: a hail of gunfire, shouting and screaming, and then quiet. "Bullets were flying through my office, over our heads, and kept going for minutes," Mercer said. "Then, I heard 'Shooter down, shooter down'. "  By that point, the base was in lockdown. A fire alarm was sounding and in buildings across the base a loudspeaker was on repeat, ordering people not to leave their offices. Across the site, doors were locked and blinds pulled down. Some had barricaded rooms with filing cabinets and upturned tables.  "We couldn't figure out how many shooters there were," said Judie Farmer, 42, an executive assistant in nearby building 210. "I didn't know if it was a co-worker who was upset or a terrorist attack. We heard gunshots and the loudspeaker just saying 'Lockdown, shelter in place'."  Grappling with confused sightings of, apparently, other gunmen, DC police announced that it was possible there were two accomplices still at large, both armed and dressed in military uniforms. The reports sent ripples of panic through the base, until the theory was discounted, several hours later. For much of the day, no-one on the compound was permitted to move. At least one victim was airlifted from the roof of a building. Even for people far from building 197, it was a traumatic experience.  "People were shocked, having palpitations; there was a woman who thought she was having a heart attack," said Angela Kaul, 32, an attorney. "It was pandemonium." Some workers were not allowed off the base until 11pm.  In the aftermath of the shooting, one response has stood out: that of Alexis's mother. "Aaron is now in a place where he can no longer do harm to anyone, and for that I am glad," Cathleen Alexis said, her voice wavering, in an audio statement recorded by CNN at her home in New York.  "I don't know why he did what he did, and I'll never be able to ask him why."  That problem applies to the FBI too. Without a suspect to interview or a written note that would account for his actions, they are asking behavioural analysts to venture some kind of answer. Teams of investigators have been dispatched across the country to interview people who knew Alexis. Officials with knowledge of the investigation said they are looking deep into his past – delving as far back as his childhood in New York – in an effort to piece together some kind of explanation for his actions.  Alexis has been described as a heavy-drinking, sometimes violent figure, estranged from his family and obsessed with violent computer games and firearms; and as an upbeat, polite character, with an insatiable curiosity for Thai culture and language. For three years he often worshipped at a Buddhist temple; he was studying part-time for an aeronautical degree.  But he was also prone to erratic behaviour. When attempting to explain an outburst to police in Seattle in 2004, Alexis's father explained that his son's behaviour stemmed from the September 11 terrorist attacks, when he was involved in rescue efforts. No other record of Alexis's involvement in the aftermath of the attacks has emerged.  In 2007, Alexis enrolled in the navy reserves at Fort Worth, Texas. He attained the rank of petty officer third class before his military career was cut short in 2011, after several incidents of misconduct.  He was arrested at least three times in his adult life. Two of the incidents involved guns. During the first, in Seattle in 2004, he shot the tyres of a construction worker's car. In 2010 in Texas, he fired through a ceiling, into his upstairs neighbour's apartment, after a dispute over noise.  The Pentagon has since conceded that these and other incidents should have served as "red flags", and announced a large-scale security review. But Alexis was never charged with any offence and while the arrests hinted at a man unable at times to control his temper, none of the incidents were significant enough to raise more serious concerns with his superiors.  Recently, however, Alexis appears to have been exhibiting more troubling behaviour. He had been working on the navy marine corps computer systems since September 2012, on a contract held by a Florida-based company called The Experts. He spent a couple of months in Japan, took some time off work, and then began a new contract on the east coast in July.  Alexis was granted a secret-level security clearance after he joined the navy in 2007. The low-level clearance granted to Alexis typically remains valid for 10 years, and is designed to identify individuals at risk of compromising security. Mental health issues are not a disqualifying factor. Nevertheless, it appears that his employers at The Experts began to express concern about his wellbeing recently. The New York Times reported on Friday that on 7 August an employee at The Experts called the hotel where Alexis was staying in Newport, Rhode Island, while on a contract there. A hotel log quoted by the Times said: "Brenda from The Experts called re: Mr Alexis in 407. She explained that he is unstable and the company is bringing him home. She asked me to check his room (it was vacant), and check him out."  In the early morning on the same day, Alexis called police to his room at the Marriott Residence Inn. He told the officers that a person he had argued with at an airport in Virginia had dispatched two men and a woman to keep him awake by sending vibrations into his body. He believed the vibrations were going to harm him, and had moved hotels twice in an effort to escape them.  His tormentors would not leave him alone, Alexis said. Convinced that voices were sending vibrations into his body "with some sort of microwave machine", he had dialled 911 and asked for help. One police officer asked Alexis what the voices were saying, but he did not reply. A police report was filed and passed to the local navy station, although it appears no action was taken.  A couple of weeks later, on 18 August, Alexis visited a Buddhist temple in Raynham, Massachusetts, and – speaking in Thai – told the monks he needed somewhere to stay. Asked why he could not stay in a hotel, Alexis said that the voices would bother him there. The monks allowed him to stayed the night in a nearby school building, leased by the temple, according to the Boston Globe.  Over the following week, Alexis twice visited emergency medical rooms run by the Department of Veterans Affairs, first in Providence, Rhode Island, and then in Washington DC. He complained of insomnia. On both occasions he was given small doses of the antidepressant Trazodone, to help him sleep.  Doctors said Alexis looked alert and aware of his surroundings as they asked him routine questions. What he anxious, or depressed? Had he thought of harming himself? Had he considered harming anyone else? He answered no.  Alexis checked in to the Residence Inn in Washington on 7 September, for the start of his spell working on computers at navy yard. The following Saturday, 14 September, he visited the Virginia shooting range. Two days later, he drove into the yard in a rented car, flashed his security pass to enter building 197 and headed straight to the men's bathroom on the fourth floor, a black bag slung over his shoulder.  Investigators now believe Alexis assembled his shotgun inside. When he emerged, he immediately started "hunting people to shoot", according to James B Comey, the director of the FBI.  A few hours later, when Aaron Alexis's name was made public, there was a familiar ritual at Sharpshooters. Whenever there is a prominent murder, staff at the gun shop log onto their database to see if the killer was one of their customers. "If people are feeling upset out there, they don't know what some of us are going through, knowing that yes, he bought a shotgun from us," said an employee of the store, who did not want to be named.  Alexis had seemed "as normal as you or me having this conversation now" when he bought his shotgun, the staff member said.  "You spend time with this person. They're asking you questions and you're answering them, showing them the shotgun they're looking for – and then, two days later, surprise."
With no suspect to interview and no pre-written note to examine, the FBI is determined to put together some kind of picture to explain the navy yard shooter's actions