U.S. authorities have identified the gunman who killed six worshippers at a Sikh temple in Wisconsin as a former American soldier.

Investigators said Monday the shooter, who was killed by police, was 40-year-old Wade Michael Page, who served in the U.S. Army for about six years in the 1990s before being discharged for misconduct. They said he had numerous tattoos, including one that said “9-11,” the date of the 2001 terrorist attacks in the United States.

The shooting Sunday morning near the city of Milwaukee is being investigated as an act of domestic terrorism. Sikhs, who wear turbans and grow beards, have been mistaken in the United States for Muslims and sometimes targeted for hate crimes, including a Sikh who was killed in Arizona four days after the 2001 terrorist attacks.

A U.S. civil rights group, the Southern Poverty Law Center, described Page as a “frustrated neo-Nazi” and leader of a racist white supremacist band called End Apathy.

U.S. President Barack Obama said “soul searching” was needed on how to reduce violence in America. He said Americans would “recoil” in shock if it turns out the shooter was motivated by ethnic hatred.

Police searched Page's apartment for clues to the shooting, but have yet to determine a motive.

Indian Prime Minister Manmohan Singh, himself a Sikh, said he was shocked and saddened by the assault, calling it a “dastardly attack.” India is home to a majority of the world's Sikhs.

“It is a very sad occasion, senseless killings of people who gathered together in a religious congregation. That it has happened in very tragic circumstances is something which makes me enormously sad. I hope the American authorities would investigate who were behind this dastardly attack on innocent devotees and that they will ensure that such costly events do not take place.”

Authorities say the gunman killed the six worshippers — five men and one woman, ranging in age from 39 to 84 — before he was shot dead by police.

Investigators said three others were hospitalized in critical condition with severe gunshot wounds, including a policeman who was shot eight or nine times before a second policeman killed Page.

Sikhism is a monotheistic religion founded more than 500 years ago in the Punjab district of what is now India and Pakistan.

Authorities say the worldwide population of Sikhs is 20 million, with 400,000 to 1 million Sikhs in the U.S.