The officer was wounded but "returned fire, and that shooter was put down," said Bradley Wentlandt, the police chief in nearby Greenfield, who briefed reporters. Investigators who picked through the building afterward found four bodies inside the temple and two other victims outside, plus the gunman, Wentlandt said.
At least three wounded were being treated at Milwaukee's Froedtert Hospital, spokeswoman Carolyn Bellin told CNN. All three were in critical condition.
Family members gathered outside the temple, or gurdwara, told CNN affilliate WTMJ that survivors were being taken to a nearby bowling alley for questioning. One man told the station that his father had been shot several times when he attempted to tackle the gunman; others recounted relatives hiding in closets.
Temple spokesman: Hostages held inside
Police: Temple shooter presumed dead
But Darshan Dhaliwal, one of the gurdwara's leaders, said Sunday afternoon he wasn't sure how many were hurt.
"Our dear ones and near ones are injured and hurt, and we are trying to find out what happened," Dhaliwal told WTMJ.
And another man told the station, "Nobody's angry here. We're just confused. Was this a random act? Was this directed at us because of the way we look?"
The Sikh religion originated in northern India
Sikhs believe in one god, in reincarnation and equality among all people regardless of race or religion. Sikh men are identifiable by their beards and turbans, but often confused for Hindus or Muslims. There are about 25 million Sikhs in the world, according to the Sikh American Legal Defense and Education Fund, 700,000 of which live in the United States.
Though early reports had suggested there may have been more than one attacker, Wentlandt said officers had not identified any other gunmen. As the search went on, utility crews cut electric and gas service to the building at the request of authorities, said Cathy Schulze, a spokeswoman for area utility We Energies.
The police officer wounded in the assault is a 20-year veteran who was shot multiple times, Wentlandt said, but he was expected to survive. The officer was sent to the Sikh Temple of Wisconsin in Oak Creek, south of Milwaukee, after a 911 call about 10:25 a.m. (11:25 a.m. ET).
State Rep. Josh Zepnick, who represents much of the Milwaukee area's Sikh community, said he was "torn to shreds" by the attack.
"It's a very peace-loving community that has successfully integrated and assimilated into the metropolitan Milwaukee area," Zepnick told CNN afilliate WTMJ. He said he was a friend of the temple's president, whose status was unknown Sunday afternoon.
And state Rep. Mark Honadel, who represents the area, called the attack "craziness."
"Unfortunately, when this type of stuff hits your area, you say to yourself, 'Why?' But in today's society, I don't think there's any place that's free from idiots."
The temple has a congregation of 250 to 400, according to its website.
"I just want to say this temple was built a number of years ago and there have never been any problems with this temple," Oak Creek Alderman Dan Jakubczyk said. "They've been a plus to this city and to my district."
CNN's Marlena Baldacci and Matt Smith contributed to this report