The gunman, identified as Wade Michael Page, shot dead six people and seriously wounded three, including a police officer, at the Sikh Temple of Wisconsin on Sunday as worshippers prepared for religious services. Police shot dead the gunman.
Although the identity of the tall, bald, white suspect in his 40s was not officially released, officials confirmed the suspect as Page, 40, a former soldier.
Page at one time was stationed at Fort Bragg, North Carolina and legally owned the gun that was used in the shooting. Authorities said they were treating the attack as an act of domestic terrorism.
Page had been a member of the racist skinhead band End Apathy, based in North Carolina, in 2010, Heidi Beirich, director of the intelligence project at the Southern Poverty Law Centre in Montgomery, Alabama, said.
She said Page also tried to buy goods from the National Alliance, a neo- Nazi group, in 2000.
“That’s all we know about Wade. We are still digging through our files,” she said.
Police were searching an apartment at a duplex in the neighbourhood of Cudahy near Milwaukee, presumed to be the residence of the gunman.
Generators and floodlights were set up along the street and a bomb squad was on the scene.
The names of the victims were not made public pending notification of relatives, although members said the president of the congregation and a priest were among the victims.
Oak Creek Police chief John Richards told CNN the suspect had a military background but gave no more details.
The wounded police officer had been shot eight or nine times in the face and extremities at close range with a handgun.
None of the wounds were life-threatening.
Authorities said the gunman had used a 9mm semi-automatic pistol, which was recovered at the scene. They were trying to track the origin of the weapon.
Wisconsin has some of the most permissive gun laws in the country.
It passed a law in 2011 allowing citizens to carry a concealed weapon.
Jagjit Singh Kaleka, the brother of the president of the temple, who was among the six Sikhs killed, said he had no idea what the motive was for the attack.
“But we know the more assault weapons we distribute the more situations like this we will have,” he said.
The attack came just over two weeks after a gunman killed 12 people at a cinema in Aurora, Colorado, where they were watching a screening of new Batman movie The Dark Knight Rises.
American Sikhs said they have often been singled out for harassment, and occasionally violent attacks, since the 9/11 attacks because of their colourful turbans and beards.
The 9/11 attacks were carried out by Muslims linked to the al Qaeda militant group led by Osama bin Laden.
Sikhs are not Muslim but many Americans do not know the difference, said members of the Sikh community.