The suspected gunman, identified as Wade Michael Page, who killed six people in a shooting rampage at a Gurdwara in Wisconsin, was a "psychological operations specialists" at in the US Army before being dismissed in 1998 after six years of service.
Page, 40, is reported to have been discharged from the Army in 1998 for "patterns of misconduct," officials said. The police are still trying to determine the motive of his killing, but according to the Southern Poverty Law Center, he is the former leader of a neo-Nazi music group called 'End Apathy'.
The police continued to search his apartment. Authorities said they are treating the attack as an "act of terrorism".
An agent from the Bureau of Alcohol, Tobacco and Firearms told ABC News that the shooter had a 9/11 tattoo on his arm.
The news channel also cited unnamed sources who alleged the shooter was possibly a "skin head" or "white supremacist".
According to officials, Page received his basic training in Fort Sill, Oklahoma before being moved to Fort Bliss in Texas.
Page was a qualified parachutist who received a commendation medal, five achievement medals, two good conduct medals, the National Defense Service Medal and a Humanitarian Service Medal, officials said.
"Temple shooting was not simply assault on congregation of Sikhs; it was an offense against peaceful, innocent people," the top Democratic leader, Nancy Pelosi, twitted.
In a blog posting, the Southern Poverty Law Center said in 2010, Page, then the leader of the band End Apathy, gave an interview to the white supremacist website Label 56.
He said that when he started the band in 2005, its name reflected his wish to "figure out how to end people's apathetic ways" and start "moving forward". "I was willing to point out some of my faults on how I was holding myself back," Page said.
Later, he added, "The inspiration was based on frustration that we have the potential to accomplish so much more as individuals and a society in whole."
He did not discuss violence in the interview, the blog wrote.
Page, during the interview, also told the website that he had been a part of the white power music scene since 2000, when he left his native Colorado on a motorcycle.
He attended white power concerts in Georgia, North Carolina, West Virginia and Colorado.
At various times, he said, he also played in the hate rock bands Youngland (2001-2003), Celtic Warrior, Radikahl, Max Resist, Intimidation One, Aggressive Force and Blue Eyed Devils.
End Apathy, he said, included "Brent" on bass and "Ozzie" on drums; the men were former members of Definite Hate and another band, 13 Knots, said the blog which also posted a picture, which could be that of Page.