The Federal Bureau of Investigation (FBI) announced on Monday that the assailant was now the subject of a "domestic terrorism" investigation.

"We are looking at ties to white supremacist groups," Special Agent Teresa Carlson, head of the FBI's Milwaukee office told reporters.

"No law enforcement agency had any reason to believe he was plotting anything," she said, noting that the FBI did not have an active file on the gunman before the incident. Carlson added that authorities were interviewing the gunman's family and associates searching for a motive behind the shooting.

Wade Michael P. was identified as the gunman who opened fire on worshippers attending a service in Milwaukee near Wisconsin on Sunday. The 40-year-old reportedly burst into the Sikh temple with a 9mm handgun and several magazines of ammunition. He was subsequently killed at the scene during a shootout with police.

P. had served as a US military "psychological operations specialist" from April 1992 to October 1998.

A civil rights group, the Southern Poverty Law Center, branded him on Monday as a "frustrated neo-Nazi who had been the leader of a racist white-power band."

Meanwhile the SITE Intelligence Group, which monitors extremists, described him as an active skinhead. SITE said it had found evidence to suggest he was a member of the "Hammerskins Nation," a group that describes itself on its website as a "leaderless group of men and women who have adopted the White Power Skinhead Lifestyle."

Obama calls for 'soul-searching'

Authorities have said that the weapon P. used was purchased legally at a gun store in Milwaukee. The report is likely to fuel further debate on US gun laws and put added pressure on US President Barack Obama and his rival Mitt Romney to address the issue before the November 6 presidential election.

Commenting on the shooting on Monday, Obama said Americans need to do more "soul-searching" to find ways to reduce violence.

"All of us recognize that these kinds of terrible, tragic events are happening with too much regularity," he said at an unrelated White House bill-signing ceremony, when asked whether further gun control measures were needed.

The shooting comes just over two weeks after a gunman killed 12 people at a theater screening of the new Batman movie in Aurora, Colorado.

ccp/jlw (AFP, Reuters, AP)

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