Witnesses said the shooter who attacked a Sikh temple in the US looked like he had a purpose and knew where he was going. Authorities on Monday scrambled to find a motive for the attack that left seven people, including the gunman, dead and three critically wounded. Police called it an act of domestic terrorism.

Terrified congregants ran for cover on Sunday morning when the shooting began at the Sikh Temple of Wisconsin. The gunfire finally ended in a shootout between the gunman and police outside.

Satpal Kaleka, wife of the temple's president, Satwant Singh Kaleka, saw the gunman enter, according to Harpreet Singh, their nephew.

"He did not speak, he just began shooting," said Singh, relaying her description.

Worshippers said they had never seen the 1.83-meter tall bald man at the temple before.

Federal authorities described the suspect as a white man in his 40s, but neither provided further details nor suggested a possible motive.

"We never thought this could happen to our community," said Devendar Nagra, 48, whose sister escaped injury by hiding as the gunman fired in the temple's kitchen. "We never did anything wrong to anyone."

Late on Sunday, the investigation appeared to move beyond the temple as police, federal agents and the county sheriff's bomb squad swarmed a nearby neighborhood, evacuated several homes and searched a duplex. Bureau of Alcohol, Tobacco, Firearms and Explosives agent Tom Ahern said warrants were being served at the gunman's home. Residents were allowed to return on Monday.

Oak Creek Police Chief John Edwards said police expected to release more information on Monday. He said the FBI will lead the investigation because the shootings are being treated as domestic terrorism, or an attack that originated inside the US.

"While the FBI is investigating whether this matter might be an act of domestic terrorism, no motive has been determined at this time," Teresa Carlson, special agent in charge with the agency's Milwaukee division, said in a statement on Sunday night.

The gunman suspected of the attack and killing was a former US army "psychological operations specialist", the Pentagon said on Monday.

Wade Michael Page, who was himself shot dead by police during the incident, was a 40-year-old who had served between April 1992 and October 1998, ending his career at Fort Bragg, North Carolina.

During a chaotic few hours after the first shots were fired around 10:30 am, police in tactical gear and carrying assault rifles surrounded the Sikh Temple of Wisconsin with armored vehicles and ambulances. Witnesses struggled with unrealized fears that several shooters were holding women and children hostage inside.

Edwards said the gunman "ambushed" one of the first officers to arrive at the temple as the officer tended to a victim outside. A second officer then exchanged gunfire with the suspect, who was fatally shot. Police had earlier said the officer who was shot killed the suspected shooter.

The wounded officer was in critical condition along with two other victims on Sunday night, authorities said. Police said the officer was expected to survive.

Tactical units went through the temple and found four people dead inside and two outside, in addition to the shooter.

The Milwaukee County Medical Examiner's Office has received the bodies of the seven dead and autopsies was scheduled to be conducted on Monday morning, Medical Examiner spokeswoman Karen Domaglski.

Jatinder Mangat, 38, another nephew of the temple's president, said his uncle was among those shot, but he didn't know the extent of his injuries. When Mangat later learned people had died, he said "it was like the heart just sat down".