Police in Wisconsin are calling a shooting at a Sikh temple that killed seven people including the gunman Sunday morning an act of “domestic terrorism.”

A gunman opened fire around 10:30 a.m. local time during a service at The Sikh Temple of Wisconsin in Oak Creek.

During a news conference on Sunday afternoon, Police Chief John Edwards said he could not provide the names and ages of the victims.

He said police are treating the shooting as an act of “domestic terrorism,” which is an act of terrorism perpetrated by someone within the confines of the United States.

“It’s not from another country,” Edwards said. “It’s from within the United States.”

Because it is considered an act of domestic terrorism, the FBI will be aiding in the investigation, said Edwards.

Late Sunday, a vague description of the shooter emerged: a heavy-set white man in his thirties or forties. However, a motive for the shooting remained unclear.

"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, FBI Special Agent in Charge with the agency's Milwaukee division, said in a statement issued Sunday night.

On Sunday evening, police evacuated a number of homes in the nearby neighbourhood of Cudahy, which is about nine kilometres northeast of the temple.

The Associated Press reported that police roped off four blocks that included a mix of duplexes and single-family homes.

In addition to local police officers, FBI agents attended the scene with an armoured truck, a trailer and other equipment. Reports suggested police had roped off the area as they searched the suspect’s home.

Meanwhile, Edwards told reporters that investigators located weapons at the scene of the shooting, but had to wait for an evidence team to enter the site to recover them.

Police said four of the deceased were inside the temple and the three others, including the shooter, were outside the temple, which is located about 20 kilometres south of Milwaukee.

Three others were shot, including one police officer who received multiple gunshot wounds after being “ambushed” by the gunman, said Edwards. The wounded officer was rushed to a local hospital where he underwent surgery. Police expect him to survive.

Another police officer fatally shot the gunman.

The mayor of Oak Creek, Steve Scaffidi, spoke to reporters after Edwards.

Scaffidi said the city is outraged at the violent act and is grieving with the families of the victims.

He thanked the police officers who helped to contain the shooting, and in doing so “made a very dangerous situation a little bit better and probably saved lives.”

Scaffidi said the city will co-operate with all officials as the investigation proceeds.

Dr. Lee Biblo, the chief medical officer at Froedtert Hospital, told reporters that doctors are caring for the three injured victims, including the wounded officer.

Biblo said all three victims are adult males and all remain in critical condition.

One of the victims suffered injuries to the abdomen and chest, one to the extremities and face and another suffered injuries to the neck, said Biblo.

U.S. President Barack Obama released a statement shortly after the shooting, expressing his support for the families of the victims and the survivors.

“At this difficult time, the people of Oak Creek must know that the American people have them in our thoughts and prayers and our hearts go out to the families and friends of the people who were shot and wounded,” said the statement.

The statement also praised the contributions Sikh-Americans have made to the country.

“As we mourn this loss which took place at a house of worship, we are reminded how much our country has been enriched by Sikhs who are a part of our broader American family,” read the statement.

Worshippers flee shooting

Earlier, witnesses at the scene described an ongoing hostage situation, which had police trying to determine if more than one gunman was involved in the attack.

“At this time we have not indentified any additional gunmen,” said Greenfield Wisconsin Police Chief Brad Wentlandt said during a news conference held earlier at the scene.

Police received an unintelligible 911 call at 10:25 a.m. local time indicating there was a shooting taking place inside the temple.

Police said a religious service was going on at the temple at the time of the shooting.

Witnesses told the Associated Press that police used two armoured trucks to transport the worshippers from the temple to a nearby bowling alley. There, police interviewed the witnesses and paramedics attended to the wounded.

Many Sikhs in the U.S. worship on Sundays at a temple. A typical service includes a meditation and singing in a prayer room.

After the service worshippers gather to share a meal. The meal is open to all community members, regardless of their religious beliefs.

Gurpreet Kaur, 24, said her mother and a group of other women were preparing the meal in the temple’s kitchen when the gunman entered and started firing. Kaur said her mother felt two bullets fly by her as the women fled to the pantry.

Kaur said she believes her mother suffered shrapnel wounds.

Amardeep Kaleka said his mother was also inside the temple at the time of the shooting. She hid in a closet when shots rang out.

“When she starting hiding in the closet she didn’t know how many people were out there,” said Kaleka.

Jatin Der Mangat, of Racine, said his uncle Satwant Singh Kalekawas was one of those shot, but he didn't know the extent of his injuries. Kalekawas is also the temple’s president.

"This shouldn't happen anywhere," Mangat said.

Sunday’s shooting comes barely two weeks after a public shooting in a movie theatre claimed the lives of 12 people in Aurora, Colo.

With files from The Associated Press