The gunman who killed worshipers at a Sikh temple in Wisconsin on Sunday had become deeply embedded in the white-supremacist music scene and was well known to anti-hate watchdog groups, one of which said it had been tracking the 40-year-old for more than a decade.

In 2000, Wade Michael Page, an Army veteran, sold everything he owned aside from his motorcycle and journeyed from Colorado, eventually settling in rural North Carolina. He joined prominent “white power” rock bands. And over time, he became frustrated with what he viewed as “people’s apathetic ways” and the lack of “strict discipline in our sick society,” according to an interview he conducted with his record label.

Although authorities have not revealed a motive for the shootings, law enforcement officials said Monday that they think Page acted alone when he opened fire at the Sikh Temple of Wisconsin in this Milwaukee suburb, killing six and wounding three others — including a police officer — before police fatally shot him.

The rampage, coming just two weeks after a mass shooting at a movie theater in Aurora, Colo., forced the nation to grapple with yet another incident of horrific violence, this one aimed at a religious group whose low-key profile in this country added to the mystery of the attack. The assault Sunday put a spotlight on a little-known but vibrant — and sometimes violent — music subculture, according to watchdog groups.

“There is a whole underworld out there of white supremacist music of which the public is almost entirely unaware,” said Mark Potok, senior fellow at the Southern Poverty Law Center, which first flagged Page’s connection to hate groups in a blog post Monday. The group has been monitoring Page since 2000, when he began playing for bands with names such as Max Resist, Blue Eyed Devil and Intimidation One.

“This guy was in the thick of the white-supremacist music scene,” Potok said. “He was not a fringe player. He was well known in the scene and played in some of the best-known bands.”

Although there is no evidence that Page harbored specific resentment toward Sikhs, watchdog groups and Sikhs say it is likely that he confused the religion with Islam, because Sikh men wear beards and turbans.

Sikhism, a monotheistic religion founded more than 500 years ago in northern India, is the fifth-largest organized religion in the world. Followers, who revere a lineage of 10 gurus, have led a relatively peaceful existence in this country, although they have occasionally been the targets of violence since the Sept. 11, 2001, attacks.

Officials said Page served in the Army from 1992 to 1998, and was stationed at Fort Bliss in Texas and Fort Bragg in North Carolina. He worked as a missile-system repairman and then as a psychological operations specialist before being discharged because of a “pattern of misconduct.”

In 2000, the Colorado native hit the road with his backpack and motorcycle, he said in the 2010 interview with Maryland-based Label 56. His wanderings at one point led him to Georgia to attend “Hammerfest,” an annual white-power music festival that the Anti-Defamation League calls “a virtual Woodstock of hate rock.”

In North Carolina, he remained active in the white-power music community, as a guitarist and vocalist. He roomed and played music with Brent Rackley, a member of the Confederate Hammerskins, part of a larger organization that was once “the top dog in the skinhead world” and probably numbers in the hundreds, Potok said.

Reached by phone in North Carolina, Rackley’s father, Joseph Rackley, declined to discuss his son but said he was stunned by the shootings.

“I’m freaked out about the incident itself,” Joseph Rackley said. “It is just senseless and any other word you’d like to use to describe it. I hate it for the survivors of those who lost someone. I have nothing but sympathy for them.”

Page worked as a solo truck driver from 2006 to 2010 for Barr-Nunn Transportation, a trucking company based in Iowa. But he was fired in August 2010 after being cited in North Carolina for driving while impaired by alcohol or some other chemical substance.

The company said in a statement that he was driving his own vehicle in North Carolina at the time of the citation, and that he refused to submit to a blood alcohol test when pulled over.

After losing his job, Page apparently ran into financial trouble. Public records show that his home, in a rural part of Fayetteville, was foreclosed on in January. Page had bought the house for $165,000 in 2007, refinanced his mortgage two years later and fallen far behind in payments.

Debbie Tanna, spokeswoman for the Cumberland County Sheriff’s Department in Fayetteville, confirmed that the department had a few interactions with Page while he lived there. He was issued five gun permits on May 5, 2008, although it is not known whether he bought weapons.

Authorities issued a warrant for his arrest in October 1997 after he wrote a bad check.

It’s unclear why Page moved to Wisconsin. For about the first half of this year, he lived in a two-story apartment in South Milwaukee with his girlfriend and her son, said David Brown, who lived a floor below them. Page was not a warm neighbor, he said.

“He wasn’t friendly. He wasn’t outgoing at all,” Brown said, adding that when he would greet Page, “he’d just shrug and walk off.” Sometimes Brown would see him in the basement lifting weights. “Just him and weights and a lot of moaning.”

Sometimes Brown would see Page carrying an instrument, perhaps a guitar or a keyboard.

“The only time he had a little bounce in his step was when he had a music thing and was heading out. I understand that. Shooting people, I don’t understand,” Brown said.

A woman who answered the phone at a number listed to Page’s grandparents in Littleton, Colo., identified herself as Page’s grandmother. At times breaking into tears, she described how impossible the shootings were to believe.

“He was just a nice person,” she said, then gasped, adding: “I can’t understand him taking six other people’s lives.”

Two weeks ago, she said, her grandson inexplicably sent a bouquet of red roses to her home.

Police on Monday identified Paramjit Kaur, a 41-year-old woman, as one of those killed. The other victims, all men, were identified as Sita Singh, 41; Ranjit Singh, 49; Satwant Singh Kaleka, 62; Prakash Singh, 39; and Suveg Singh Khattra, 84. Singh is a common surname for men in the Sikh religion. The wounded officer was identified as Lt. Brian Murphy, 51, a 21-year veteran of the department. He is in critical condition, Oak Creek Police Chief John Edwards said.

On Monday, members of the Sikh community here in Oak Creek wept after the victims’ names were read during a news conference. One man wiped the tears from the face of another who had sunk to the floor during the event at the police station.

Another asked FBI Special Agent Teresa Carlson how many people like Page are scattered across the country. “That is the problem,” she replied. “Nobody knows.”

The attack jolted Internet message boards trafficked by white supremacists, some of whom urged more, similar actions. SITE Intelligence Group, which monitors radical groups on the Web, reported Monday a flurry of activity on racist message boards, including one thread exhorting the community to “stop talking and start doing.”

The record label that released Page’s music and published the interview called the attack a “tragedy,” saying in a statement that it had worked hard to be “positive.”

“Label 56 is very sorry to hear about the tragedy in Wisconsin and our thoughts are with the families and friends of those who are affected,” it said. “Please do not take what Wade did as honorable or respectable and please do not think we are all like that.”

Leonnig and Somashekhar reported from Washington. Julie Tate in Washington contributed to this report.