The perpetrator was identified on Monday as 40-year-old Wade Michael Page, a "psychological operations specialist" who served in the U.S. army from 1992 to 1998. He was discharged from the military after being demoted, officials say.
Page died in a shootout with police outside the suburban Milwaukee temple on Sunday. Just two weeks after 12 people died in a movie theatre rampage Colorado, this shooting claimed the lives of seven people, including the gunman.
Police announced later Monday they were looking for a second "person of interest," distributing a photo of an unknown white man to the media at a news conference.
"This individual showed up at the scene after the shooting," said John Edwards, Oak Creek police chief.
Witnesses said the man looked "suspicious" and he "left the scene before anyone could ascertain what he was doing there," he said.
Three others were seriously wounded in Sunday's shooting spree, including a responding police officer shot several times by the gunman, in what investigators are classifying as an act of domestic terrorism. That means law enforcement agencies consider the crime politically motivated, and have called in the FBI to investigate.
President Barack Obama said at the White House that he's "heartbroken" by the latest mass shooting.
"All of us recognize that these kinds of terrible, tragic events are happening with too much regularity for us not to do some soul-searching, and to examine additional ways that we can reduce violence," he said.
"What I want to do is bring together law enforcement, community leaders, faith leaders, elected officials at every level to see how we can make continued progress."
But Obama stopped short of advocating tougher gun control laws. Indeed, the latest shooting — like the one in Aurora, Colo., on July 20 — has done little to renew a push for more restrictive gun ownership legislation from politicians on either the left or right.
In an interview with the Huffington Post conducted as news was breaking about the events in Wisconsin, Nancy Pelosi, minority leader of the U.S. House of Representatives, held out scant hope that tougher gun laws might result.
Despite the spate of mass shootings, Pelosi said Republicans and some Democrats don't believe in restricting gun rights.
"The votes aren't there for gun control," she said. "We certainly aren't going to be able to do it in this Congress, and I don't know that we would be able to do it in a Democratic Congress because it takes a lot of votes to go down that path."
The Southern Poverty Law Center, a group that probes hate crimes, said on its website Monday that Page was a frustrated neo-Nazi and one-time frontman of a racist white-power band known as End Apathy.
The centre said it had been tracking Page for years, and in 2000 learned he'd attempted to buy goods from the National Alliance, America's biggest neo-Nazi group at the time.
They pointed to an interview Page had conducted with the white supremacist website Label 56 two years ago.
"The inspiration was based on frustration that we have the potential to accomplish so much more as individuals and a society in whole," he said of the band and its name.
Page also provided details of a series of additional "white power" bands he'd played for, with names that included Intimidation One, Aggressive Force and Blue Eyed Devils.
The MySpace page of someone named Wade Michael Page features photos of a heavy-set, heavily tattooed white man with a shaved head on stage with a guitar.
Witnesses at the Sikh temple described the shooter, armed with a handgun, as a bald white man with a 9-11 tattoo, prompting speculation that he was targeting Muslims, not Sikhs, and perhaps didn't know the difference.
Sikh rights groups in the United States have reported a rise in hate attacks against the community since the Sept. 11, 2001, terrorist attacks, with more than 700 reported incidents.
The long beards and turbans worn by some Sikhs often cause them to be mistaken for Muslims, a group that a segment of Americans have demonized in the aftermath of 9-11, given the attack's perpetrators were Muslim extremists.
Obama said any racially fuelled attack would be particularly abhorrent to Americans.
"If it turns out, as some early reports indicated, that it may have been motivated in some way by the ethnicity of those who were attending the temple, I think the American people immediately recoil against those kinds of attitudes," he said.
"It will be very important for us to reaffirm once again that in this country, regardless of what we look like, where we come from, who we worship, we are all one people and we look after one other and we respect one another."
But in what's becoming a sadly familiar hallmark of such shootings, one public figure laid the blame not on guns or intolerance, but on those who lack faith.
"Satanic" atheists are to blame for the Wisconsin shooting, television evangelist Pat Robertson said Monday on "The 700 Club," even though there's no indication Page was an atheist.
"People who are atheists, they hate God, they hate the expression of God," said Robertson, who once ran for the Republican presidential nomination. "They're angry with the world, angry with themselves, angry with society and they take it out on innocent people who are worshipping God."
Two weeks ago, a Republican congressman from Texas linked the Colorado shooting to the "ongoing attacks on Judeo-Christian beliefs" while also wondering aloud why no one in the packed theatre was able to return the gunman's fire.
"What really gets me, as a Christian, is to see the ongoing attacks on Judeo-Christian beliefs, and then some senseless crazy act of a derelict takes place," Louie Gohmert said in an interview with the Heritage Foundation.
"Where was God? What have we done with God? We don't want him around. I kind of like his protective hand being present."