OAK CREEK, Wisconsin - Police searched the apartment early on Monday of a gunman who killed six people at a Sikh temple in southern Wisconsin, looking for clues to his motive.A policeman called to the scene shot dead the gunman before he could fire on even more worshippers as they prepared for Sunday services at the temple in the suburb of Oak Creek, south of Milwaukee. Three other people were in hospital with grave injuries, including another policeman who had responded to the scene.Authorities said they were treating the attack as an act of domestic terrorism.The identity of the tall, bald, white suspect in his 40s was not immediately released. The names of the victims also were not made public pending notification of relatives, although members said the president of the congregation and a priest were among the victims.CNN, citing a law enforcement source involved in the investigation, said the gunman was an Army veteran.Authorities said the gunman had used a 9 mm semi-automatic handgun, 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 and had 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 US had a ban on certain assault weapons but it expired in 2004.Early on Monday, police were searching an apartment at a duplex in the Cudahy neighborhood 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 attack came just over two weeks after a gunman killed 12 people at a theater in Aurora, Colorado, where they were watching a screening of new Batman movie "The Dark Night."In January 2011, a gunman killed six people in an attack on an event by then Congresswoman Gabrielle Giffords in Tucson, Arizona. Giffords was shot in the head but survived.American Sikhs said they have often been singled out for harassment, and occasionally violent attack, since the September 11, 2001 attacks because of their colorful turbans and beards.The 2001 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, members of the Sikh community said.Some witnesses to the Wisconsin shooting said the suspect had a tattoo marking the September 11, 2001 attacks. Authorities confirmed he had tattoos but said they were not sure exactly what the tattoos illustrated.There are 500,000 or more Sikhs in the United States but the community in Wisconsin is small, about 2,500 to 3,000 families, said local Sikhs.The Sikh faith is the fifth-largest in the world, with more than 30 million followers. It includes belief in one God and that the goal of life is to lead an exemplary existence.The temple in Oak Creek was founded in October 1997 and has a congregation of 350 to 400 people."These people were going to church. Two weeks ago, it was people going to a movie. When is it going to end?" said Ray Zirkle, who came from Racine, Wisconsin with his wife to light votive candles near the site of the shooting.