The gunman who killed six people inside a Sikh temple in Wisconsin was a neo-Nazi who performed in white supremacist bands, and had once been a psychological operations specialist in the U.S. Army.

At a news conference Monday, Oak Creek Police Chief John Edwards identified the gunman as 40-year-old Wade Michael Page.

The Southern Poverty Law Center, a non-profit civil rights organization, says Page was a "frustrated neo-Nazi" who had led a white-power rock band called End Apathy since 2005.

Page told a white supremacist website in a 2010 interview that he had been part of the white-power music scene since 2000. He said his "inspiration was based on frustration that we have the potential to accomplish so much more as individuals and a society in whole.”

Page had previously joined the military in 1992 and became a psychological operations specialist, stationed in Fort Bragg, North Carolina. Sources told The Associated Press he was demoted in 1998 for being intoxicated while on duty “and going AWOL.”

He was discharged that same year.

On Sunday, Page was shot dead by police, ending a rampage that left five men and one woman dead at the Sikh Temple of Wisconsin. Three others were seriously wounded in the attack, including a police officer.

One of the slain men, 65-year-old Satwant Singh Kaleka, was praised as a hero who died trying to stop Page. During the shooting, Kaleka had found a butter knife and attempted to stab Page, but was shot twice.

The other victims included Prakash Singh, 39; Sita Singh, 41; Ranjit Singh, 49; Paramjit Kaur, 41; and Suveg Singh, 84.

Local police said Page was not known to them before Sunday's massacre. Online records show Page had a brief criminal history in other states, including pleading guilty to misdemeanour criminal mischief in El Paso, Texas, and driving under the influence in Colorado. Both incidents occurred in the 1990s.

Police say the shooter worked alone and was armed with a nine-millimetre, semi-automatic handgun and multiple magazines of ammunition, all of which was purchased legally.

However, the FBI released a photo Monday of a "person of interest" they wanted to question in the investigation.

The unidentified man showed up at the scene after the shooting and looked "suspicious," Edwards said, but left before police could interview him.

The FBI is leading the investigation because the shootings are being treated as “domestic terrorism.”

Worshippers at the temple said they had never seen the gunman before the massacre, but he seemed like he had a purpose and knew where he was going. They said he didn’t say a word as he entered the temple; he just walked in and began shooting.

"We never thought this could happen to our community," said Devendar Nagra, whose sister survived the attack. "We never did anything wrong to anyone."

American Sikhs say they have often faced harassment, and occasionally violent attacks, since the Sept. 11, 2001 terror attacks. Although those attacks were carried out by Muslims linked to al Qaeda, many Americans do not understand that Sikhs are not Muslims, Sikh leaders say.

Sikhism is a monotheistic faith founded in South Asia, more than 500 years ago. Though it is the fifth-largest religion in the world, there are only about 500,000 Sikhs in the United States.

By late Sunday evening, police and the FBI had evacuated a number of homes in the nearby neighbourhood of Cudahy, about nine kilometres northeast of the temple, where the gunman lived. They roped off four blocks that included a mix of duplexes and single-family homes.

Bureau of Alcohol, Tobacco, Firearms and Explosives agent Tom Ahern said warrants were served at the gunman's home. On Monday morning, residents were allowed to return to their homes.

The shooting began just before 10:30 a.m. local time Sunday during a regular morning service.

One of the first officers to respond to a 911 call for help was ambushed in the parking lot by the gunman as he tried to tend to two fatally wounded victims outside. The officer was shot eight to nine times.

A second officer then exchanged gunfire with the suspect, and shot the gunman dead.

The wounded officer was in critical condition, but was expected to survive.

Police in tactical gear and carrying assault rifles then surrounded the temple, amid fears that several shooters were holding women and children hostage inside. When the tactical units eventually went inside, they found four more bodies.

Dr. Lee Biblo, the chief medical officer at Froedtert Hospital, told reporters Sunday that doctors are caring for three injured victims, including the wounded officer. Biblo said all three are adult males and all remain in critical condition.

The mayor of Oak Creek, Steve Scaffidi, said the city is outraged at the violent act and is grieving with the families of the victims.

U.S. President Barack Obama released a statement expressing his support for the families of the victims and survivors and praising the contributions Sikh-Americans have made to the country.

“As we mourn this loss which took place at a house of worship, we are reminded how much our country has been enriched by Sikhs who are a part of our broader American family,” read the statement.

Sunday’s shooting came barely two weeks after a public shooting in a movie theatre claimed the lives of 12 people in Aurora, Colo.

With files from The Associated Press