Washington (CNN) -- Aaron Alexis arrived in Washington on August 25 after a tumultuous month. He'd had a run-in with a family at the airport in Norfolk, Virginia, where he accused them of laughing at him. He told police in Rhode Island that the family sent people to follow him and keep him awake by talking to him through the walls, floor and ceiling of his room and sending "vibrations into his body" via "some sort of microwave machine."
But when Benita Bell met Alexis on September 10 at a Washington hotel where they both stayed, she was struck by his friendly face.
"We crossed paths, and he had this warm smile, so we both said 'hello,' " she said of their first encounter at the Residence Inn just five blocks from the U.S. Capitol and less than two miles from the Washington Navy Yard.
Bell was in town for her work in astrobiology. He was in town for work as an information technology contractor with the government.
They spoke for about 15 minutes about work and their love of the South. She is from North Carolina. He grew up in New York but had been living in Fort Worth, Texas.
Because he was "very cordial, very engaging, very present," she gave him her phone number and said he should call if has questions about the city.
"Let's keep in touch," Alexis responded.
She knew nothing of his background -- "a pattern of misconduct" during his four years as a Navy reservist, his arrest in 2004 after an anger-caused "blackout" or the help he sought from the Veterans Affairs department for sleep issues.
Plus, his father says, Alexis was in New York on September 11, 2001, helped in the World Trade Center rescue effort and was "disturbed" by the terror attack, suffering from what the father described as post-traumatic stress syndrome.
Bell and Alexis ran into each other again on this past September 11, and she said he seemed like a different person -- "extremely tired, exhausted."
"I got to go. I got to go. I'm going to take my food up to the room," Alexis hurriedly said to her.
That was the last she saw him. Until his face was plastered across the media five days later.
In the meantime, Alexis visited a gun range in Lorton, Virginia, across the Potomac River and nearly 20 miles south, and paid $419 for a gun and two boxes of ammunition.
A routine Monday
The heat of the Washington summer had finally broken. Monday morning was cool, and clouds gently blocked the rising sun.
Driving to the Navy Yard, Alexis would have passed the pockets of low-income, low-rise apartments and row houses that are quickly being replaced by taller luxury apartments and renovated single family homes complete with granite countertops.
Passing through the fortress-like red brick walls that separate the Navy Yard from the surrounding neighborhood that bears the same name, Alexis would have flashed his contractor ID and was waved onto the base.
Alexis was among 18,000 people, many of them civilian employees or government contractors, who work at the sprawling Navy Yard complex that sits on the banks of the Anacostia River in southeast Washington near where it flows into the Potomac. He'd been working on a computer system revamp.
Investigators are still going through evidence and clues that Alexis left behind, but they'll never know what was going through his mind as he parked his rented Toyota Prius in a small parking deck across from Building 197 at 8:01 a.m.
Navy Yard employees were starting a new work week. Inside Building 197, a large rectangular structure with an open floor plan around an open atrium, workers greeted colleagues good morning, talked about the weekend, settled into desks, turned on computers, grabbed coffee, ate breakfast. It was John Weaver's birthday.
For the couple of weeks he worked at the Navy Yard on a computer system revamp, Alexis was one of those employees, seemingly like everyone else. He went to work in the morning and left at the end of the day. On this day, he was markedly different.
A minute after he parked his car, he entered Building 197, carrying a black bag that drew no special attention. He wasted no time.
He headed directly to an elevator, bypassing the first floor cafeteria and the sunlit atrium of the recently refurbished office building. He got off the elevator on the fourth floor and headed directly to a bathroom, where he quickly pulled out the contents of his bag at 8:03 a.m.
Gun Assembly Shop
The Washington Navy Yard's long, storied history dates back to 1799. It was the first U.S. Navy land outpost and served as a naval ship-building factory. During the War of 1812, when the Capitol and much of Washington was burned, the yard's commander ordered the buildings to be burned to prevent being captured.
The yard's mission changed from shipbuilding to designing and manufacturing weapons and ammunition. It was integral to Washington's defense during the Civil War.
Building 197 was constructed in 1939 and called the Gun Assembly Shop. In the bathroom, Alexis assembled his gun. Minutes later, he walked out of the restroom with his Remington 870 tactical shotgun.
Alexis had customized the gun. It had been sawed down on both ends, making it shorter and more maneuverable. He had etched "My ELF weapon" into the stock. More alarmingly, the phrase "Better off this way" was also carved into it.
He carried double-aught buckshot shells in the cargo pockets in his pants, each packed with about a dozen pellets and capable of causing tremendous damage.
Meanwhile, Denise Robinson, who works on the fourth floor of Building 197, was chatting with co-workers. The U.S. Navy Band, which is based at the yard, was playing outside on the courtyard.
Alexis fired his first shot.
It was 8:12 a.m. Weaver was at his desk, one cubicle away from the atrium. His co-worker chuckled when he said the commotion in the building sounded like someone "was skateboarding on the ceiling." Then came a loud "bang." He thought someone dropped a safe in the atrium.
After the second loud boom, Weaver knew he was under attack.
"We hear pop, pop sounds." Robinson said. "We really didn't know what was going on."
Confused and curious, Robinson stood up to see above her cubicle walls. She saw her supervisor and the gunman. In her boss' face, she saw fear. In Alexis' she saw "a cold stare."
Weaver saw that 50-year old Frank Kohler was hit. So was 73-year old John Roger Johnson.
Weaver stood up, too. He saw Alexis at the end of his row of cubicles. He pointed the gun at the row of cubicles diagonal from Weaver where a friend was sitting.
"The reason he did not see me because he was so intent on shooting her," Weaver said.
Weaver's friend "turned around while she was curled up and watched him shoot her," he said, calling her the "bravest" and "luckiest person on the face of the Earth."
'Unbelievable how many shots were going off'
The friend survived but she lost much of a finger on her right hand and the blast grazed her scalp.
"[B]its of her scalp are scattered all over her cubicle" Weaver said.
Weaver got under his desk and pulled a two-drawer filing cabinet in front of him to hide.
Alexis then walked through a corridor with copy machines on one side and kitchenettes on the other.
Weaver called 911 from his cell phone. He later said it "was unbelievable how many shots were going off" while he was on the phone.
Bertilla Lavern's supervisor, Andy Kelly, told her to get down and then pulled her into his cubicle and told her to get under the desk.
When Lavern and Kelly got out from under the desk, she found co-worker Vishnu Bhalchandra Pandit who sat "right across" from her.
He was shot in the temple, but his pulse was "strong."
"I felt him breathe," Lavern said, describing that moment as "awesome and phenomenal." She said "it's like he heard me calling to him, talking to him, praying over him, letting him know that we love him and we want him to stay with us."
Lavern and some security guards loaded Pandit to an evacuation chair and carried him downstairs.
Alexis continued to walk through the building, blasting away. On the fourth floor, he killed 58-year old Gerald Read, 62-year-old Kathy Gaarde and 53-year old Sylvia Frazier, whom Weaver said "was the nicest person in the world."
At 8:16 a.m., Alexis entered a stairwell and exited on the third floor where he began randomly shooting in all directions. At one point, he reached into his pocket for more shells.
An announcement came over the intercom and told people to evacuate the building because of a "fire emergency," witness Terrie Durham said.
Michael Arnold suffered from an early season cold. He was on the phone with his wife who called to see how he was feeling when an alarm went off. He told her he would call her back, but he didn't get to.
Weaver came out from under his desk and ran. He turned the corner and saw his supervisor, 51-year-old Mary Francis Knight, lifeless on the ground.
He kneeled over her and looked into her eyes "to see if there was consciousness, and there was none."
Weaver saw his friend who had been shot in the finger and side of the head. Weaver downplayed her injuries to comfort her, telling her he'd gotten bigger cuts playing hockey and said, "Let's get out of here."
'Get out of the building now!'
Durham was gathering his belongings. Then he began to move with more urgency when people began screaming, "Get out of the building now! Get out of the building now!"
Durham and three co-workers walked down the hall when they came within eyesight of Alexis. Unaware that Alexis was holding a gun, the foursome continued walking toward him. Alexis stood out because he wasn't moving with the same urgency as everyone else.
"I kept thinking, 'What is he doing? There's a fire in the building. We need to get out,' " Durham said.
Then Alexis opened fire in their direction. Incredibly, he missed.
As Weaver ran down the stairs, he told people to "run for your lives." Once he reached the first floor, he saw a conference room full of people, seemingly unaware of the carnage on the floors above. He told them to get out and "everybody just started scattering."
"People went into a complete panic at that point," Weaver said. He told an officer "there's a guy slaughtering people with a shotgun" on the fourth floor and they "need to get up there with a lot of weapons."
The 911 calls started pouring in. "We have an officer down, Building 197, third floor," one dispatcher said.
Alexis was now on the first floor, where he shot a guard and took his handgun.
While Alexis was on the first floor, Lavern and the security guards made it to the first floor with Pandit, who was still breathing. They heard Alexis was on the west side of the building. They escaped through an alternate exit to a street corner away from Building 197. Pandit was still breathing.
Unaware of the terror unfolding in Building 197, another worker came out to help administer CPR to Pandit, who he thought had suffered a heart attack. Pandit later died from his wounds.
Alexis returned to the third floor. Out of shotgun ammunition, he used the guard's Beretta handgun.
Seven minutes after the first 911 call, officers from Washington Metropolitan Police Department, Naval Criminal Investigative Service and the U.S. Park Police began arriving on the scene and got into what they described as a "running gun battle" with Alexis. He ducked down hallways and ran in and out of offices and conference rooms. Several times authorities thought he was down, but he'd come out firing.
'One of the worst things we've seen'
MPD Chief Cathy Lanier called the crime scene "one of the worst things we've seen in Washington."
"As officers entered the building, they were, you know, making transmissions in and keeping command informed as to what they were coming across as they went through," she said. "Multiple victims. There was gunfire still going on."
By 8:45 a.m., the first tactical teams, better equipped to handle this kind of emergency, began arriving. Not long after that, Alexis was finally down for good.
At 9:30 a.m., 90 minutes after Alexis drove on to the base, he was pronounced dead. He had killed 12 Navy Yard workers and wounded eight others.
In the following hours, police searched for two other possible gunmen. By afternoon, one of those men was determined not to be connected. By that night, police were sure Alexis acted on his own. The FBI said the confusion possibly was a result of people with weapons in hand trying to stop Alexis.
Finally, they cleared the building. Workers left the yard and the reality of what had happened began to set in.
While investigators worked to put together the pieces of what took place on that horrific Monday morning, the Navy Yard complex reopened. But Building 197 remains closed. It could be weeks before workers are able to return to their cubicles and offices.
It will be even longer before their Mondays once again feel normal.
CNN's Pamela Brown, Dugald McConnell, Evan Perez and Brian Todd contributed to this report