US President Barack Obama says America is heart-broken by the killing of Sikhs in a Gurdwara there and asked his countrymen to do some soul-searching and examine ways to reduce violence.

A former US Army veteran believing in the white supremacist movement had opened fire on worshippers at a Gurdwarda in Oak Creek in Wisconsin on Sunday morning, killing six people before he was shot dead.

Obama said if it turned out, as some early reports indicate, that it might have been motivated in some way by the ethnicity of the worshippers, the American people immediately recoil against that kind of attitude.

President Obama said it would be very important for the Americans to reaffirm that regardless of looks, origin and beliefs, they were all one people and look after one another and respect one another.

Meanwhile, the FBI is looking into the links of the gunman involved in the shooting to white supremacist groups with a history of hate towards Jews and black people but ruled out participation of a second person in the attack.

As more details emerged former US army psychological-operations specialist about Wage Michael Page's 'neo-Nazi' leanings, the FBI today intensified its probe to ascertain the motive behind the killing of the Sikh worshippers.

Page's neighbours said he rarely left his one-bedroom house where he lived alone, and never made eye contact, but civil organisations which monitored his actions, described the 40-year-old as a “frustrated neo-Nazi” who had been leader of a racist white-power band.

The FBI said they were looking into Page's ties to white supremacist groups but insisted there were no prior warning signals that could have led investigators to believe he was plotting something so vicious.

Head of the FBI’s Milwaukee office Special Agent Teresa Carlson said the gunman was the subject of a domestic terrorism probe. The FBI also ruled out the involvement of a second person in the Sunday shooting, hours after releasing the picture of a person of interest.

Officials cleared the man after interviewing him and affirmed that the slaying was the handiwork of a lone gunman.

Earlier on Monday, the FBI said they were trying to identify a suspicious man who arrived at the scene after the shooting and released a photograph of him, asking for the public's help.

The US has witnessed a number of such shooting incidents and the latest carnage came even as the nation was still reeling from a mass shooting two weeks ago in which a gunman killed 12 people in a theatre in Colorado.

Earlier, India sought assurance from the US over the safety of the Indian community living in that country. External Affairs Minister SM Krishna spoke to US Secretary of State Hillary Clinton regarding the shooting incident.