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 records.
Nine people were wounded when he sprayed bullets into a classroom at the college in Roseburg, a timber town of about 20,000 people that 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 another shooter received after killing two television journalists in Virginia earlier this year.
"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," the post said. "Seems the more people you kill, the more you're in the limelight."
Harper-Mercer was in the Army for a month in 2008 before being discharged for failing to meet administrative standards, military records showed.
Eileen Sandlin, 40, a resident of the Los Angeles neighbourhood of Tarzana where the gunman's father Ian Mercer lives, said her husband, Bob, used to tutor Harper-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 said he seemed socially inept, but that there was no indication Harper-Mercer 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 fighters toting assault rifles in the Irish Republican Army, a militant group that waged a violent campaign to drive the British from Northern Ireland.
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 his parents were married in 1989, the year he was born, separated in 1990, and that their divorce was finalised in 2006. Laurel Harper was awarded custody of her son.
The Torrance-based Daily Breeze said he graduated in 2009 from Torrance's Switzer Learning Centre, a private non-profit school that says it serves students aged from third-grade to 22, most of whom have learning or emotional disorders. A Torrance police spokesman confirmed Harper-Mercer attended the centre.
It was not clear when the gunman moved to Oregon.
Former neighbour Reina Webb, 19, said she often saw the gunman wearing military-style fatigues and acting oddly around the Torrance apartment complex where he and his mother lived.
"He was a loner ... He was a grown man, but acted like a little kid," said Webb. "He would have temper tantrums and fits if he didn't get his way."
Her friend, Jason Lee, 19, said Harper-Mercer would sometimes yell at children from his red bicycle.
SEEKING LOVE ONLINE
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 "teetotaller", 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".
The user's last torrent upload was on Tuesday and was entitled "This World Surviving Sandy Hook BBC Documentary 2015," according to the website.
Get Breaking news, live coverage, and Latest News from India and around the world on NDTV.com. Catch all the Live TV action on NDTV 24x7 and NDTV India. Like us on Facebook or follow us on Twitter and Instagram for latest news and live news updates.