At least six people were killed Sunday when a gunman opened fire at a Sikh temple near Milwaukee, and the suspected shooter later died in an exchange of gunfire with police, authorities said.

Police were called to the Sikh Temple of Wisconsin in the suburb of Oak Creek, where witnesses said several dozen people were gathering for a service. Authorities found four people dead inside the temple and two outside, Greenfield Police Chief Bradley Wentlandt said.

Oak Creek Police Chief John Edwards, where the incident took place, told a news conference that the shooting rampage is being treated as a domestic act of terrorism and would be investigated by the F.B.I.

"Because of the heroic actions of our officers they stopped this from being worse than it could have been," Edwards said, adding that a 20-year police veteran underwent surgery after being "ambushed" by the shooter and was expected to recover. He confirmed that another officer shot and killed the suspect.

"It stopped a tragic event that could have been a lot worse," he said. "There was a service going on with many people at that location."

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

Sunny Singh, 21, of Milwaukee, said a friend pulled into the temple's parking lot, heard shots and saw two people fall down. The friend then saw the shooter reload his weapons and head to the temple's entrance, Singh said.

Tactical units went through the temple, and authorities do not believe a second shooter was involved, Wentlandt said.

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

Two others were wounded along with the police officer, Edwards said. He said authorities would not say any more about their investigation until Monday morning, including the names of those killed.

But it appeared the investigation had moved beyond the temple, as police and FBI agents focused on a neighborhood in nearby Cudahy. Authorities would not comment on the activity, which included evacuating several homes.

GOP presidential candidate Mitt Romney called the shootings "senseless" and joined President Obama in expressing condolences to America's Sikh community.

“Ann and I extend our thoughts and prayers to the victims of today’s shooting in Wisconsin," said Romney. "This was a senseless act of violence and a tragedy that should never befall any house of worship. Our hearts are with the victims, their families, and the entire Oak Creek Sikh community. We join Americans everywhere in mourning those who lost their lives and in prayer for healing in the difficult days ahead.”

Jatin Der Mangat, 38, of Racine, the nephew of the temple's president, Satwant Singh Kaleka, said his uncle was one of those shot, but he didn't know how serious his injuries were. He was among those waiting for news when police announced the deaths.

"It was like the heart just sat down," he said. "This shouldn't happen anywhere."

Sukhwindar Nagr, also of Racine, said he called his brother-in-law's phone and a priest at the temple answered and told him that his brother-in-law had been shot, along with three priests. The priest also said women and children were hiding in closets in the temple, Nagr said.

Wentlandt did not identify the suspect or say what might have motivated the shootings.

Sikhism is a monotheistic faith that was founded in South Asia more than 500 years ago. It has roughly 27 million followers worldwide. Observant Sikhs do not cut their hair; male followers often cover their heads with turbans - which are considered sacred - and refrain from shaving their beards. There are roughly 500,000 Sikhs in the U.S., according to estimates. The majority worldwide live in India.

Sikh rights groups have reported a rise in bias attacks since the Sept. 11 terrorist attacks. The Washington-based Sikh Coalition has reported more than 700 incidents in the U.S. since 9/11, which advocates blame on anti-Islamic sentiment. Sikhs don't practice the same religion as Muslims, but their long beards and turbans often cause them to be mistaken for Muslims, advocates say.

Newsmax staff contributed to this article.