Chris Harper-Mercer lived in Torrance, California, before moving to Winchester, Oregon, where he resided in an apartment with his mother Laurel Harper about a four-minute drive from Umpqua Community College, according to online directories.
At least nine other people were also wounded when he sprayed bullets into a classroom at the college in Roseburg, a timber town of about 20,000 people which adjoins Winchester.
On Friday, police outside the pink apartment building where he lived said his mother was not home. No one answered the phone number listed for the address, where investigators recovered two pistols, four rifles and one shotgun, in addition to six weapons the shooter took to the college.
A blog post using an email address linked to Harper-Mercer noted the publicity that another shooter received after killing two television journalists in Virginia earlier this year.
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"A man who was known by no one, is now known by everyone. His face splashed across every screen, his name across the lips of every person on the planet, all in the course of one day. Seems the more people you kill, the more you're in the limelight," the post said.
Harper-Mercer was in the army for one month in 2008 before being discharged for failing to meet administrative standards, military records showed.
Eileen Sandlin, 40, a resident of the Los Angeles neighborhood of Tarzana where Mercer's father Ian Mercer lives, said her husband, Bob, used to tutor Mercer in science and math when he was of middle and high school age.
"He was a quiet kid," Sandlin said. "I got kind of a creepy feeling from him." She added that "he seemed a bit stunted with social skills," but said there was no indication he would do what he did, and that he was never fanatical about religion.
Family members and victims told CNN and other media the gunman lined up students and asked them if they were Christian. Those who answered "yes" were shot in the head.
A photo posted on a MySpace profile belonging to someone named Chris Harper-Mercer showed a young man with a shaved head, dark-rimmed glasses and a serious expression. He was holding a long-barreled gun.
Other images uploaded to the profile depicted balaclava-clad Irish Republican Army fighters toting assault rifles, alongside the slogan "Undefeated Army."
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Harper-Mercer was born in the United Kingdom and arrived in the United States as a young boy, his stepsister Carmen Nesnick told CBS Los Angeles.
Los Angeles Superior Court records show Harper-Mercer's parents were married in 1989, the year their son was born, separated in 1990, and that their divorce was finalized in 2006. Laurel Harper was awarded custody of her son.
The Torrance-based Daily Breeze reported that he was listed in 2009 as graduating from Torrance's Switzer Learning Center, a private non-profit school that according to its website serves students aged from third-grade to 22, most of whom have learning or emotional disorders.
It was not clear when the gunman moved to Oregon.
'Seemed skittish'
"I just started noticing him the last four to six months. There's a playground out behind the apartments, and we're on the second story. I go outside to smoke and I watch for anyone," said neighbor Steven Fisher, who said he had never spoken to Harper-Mercer.
"Around 5-5:30 in the evening I'd see him out there with a couple kids. He seemed skittish, always looking over his shoulder to see if anyone was watching," Fisher said.
On Thursday, police tape had been drawn across the door of the apartment where he lived, and officers with a dog were posted outside. Investigators from agencies including the Douglas County Sheriff's Department, Homeland Security Protective Services and the FBI were also on site.
But on Friday the police presence was minimal.
In an Internet posting on the Spiritual Passions dating and social networking site, a user posted a picture that appears to be Harper-Mercer under the user name Ironcross45, a handle Harper-Mercer used as his email.
He described himself on the site as a 26-year-old, mixed-race "man looking for a woman." He said he was "not religious, but spiritual," a "teetotaler", and a conservative Republican living with his parents. Socially, he said, he was "shy at first" and "better in small groups." He described himself as "always dieting" and looking for "the yin to my yang."
The same email address linked to a Chris Harper-Mercer was also associated with the profile of user Lithium_Love on torrent sharing website KickAssTorrents.
The user wrote a blog post on the site about Vester Flanagan, the man who shot dead two reporters during a live broadcast in August before killing himself, calling the footage of the shooting "good".
"On an interesting note, I have noticed that so many people like him are all alone and unknown, yet when they spill a little blood, the whole world knows who they are," the post said.
The user's last torrent upload was on Tuesday and was entitled "This World Surviving Sandy Hook BBC Documentary 2015," according to the website.
