The gunman who killed six people in a Sikh temple in the US before being killed has been identified as a "frustrated Neo-Nazi" and army veteran.

He was identified as Wade Michael Page, assistant US attorney Greg Haanstad in Milwaukee said.

He joined the army in the early 1990s and was discharged in 1998, a US military official said.

According to the Southern Poverty Law Centre (SPLC), Page was also the leader of a white supremacist organisation who was a "frustrated neo-Nazi".

In an interview on a white supremacist website in 2010, Page claimed he was also the leader of a white supremacist band, End Apathy, which he founded in 2005 after leaving his native Colorado.

His inspiration was "based on frustration that we have the potential to accomplish so much more as individuals and as society as a whole", the SPLC said.

Officials and witnesses said the gunman walked into the Sikh Temple of Wisconsin before opening fire on a dozen people who were getting ready for Sunday morning services. He killed six. Three people were left in critical conditions.

Psy-ops specialist

Page joined the military in 1992 where he became one of the army's psychological operations (psy-ops) specialists, primarily responsible for the analysis, development and distribution of intelligence.

Joseph Rackley of North Carolina told Associated Press that Page lived with his son in 2011 in a house on his property.

Page had tattoos all over his arms, Rackley added, although he denied knowing about the gunman's white supremacist links.

Witnesses of the attack on the Sikh temple said Page did not say anything before he opened fire.

Page shot several times a police officer who was tending a victim outside the temple before the gunman was shot dead by a second officer.

Police said the attack was being treated as domestic terrorism. The FBI would lead the investigation.