The US FBI are handling the investigation into the Sikh temple shooting as a "domestic terrorism" case, Oak Creek Police Chief John Edwards said.
He also said the temple complex has been cleared, but they are still methodically searching the area.
The police chief of the city of Greenfield, Brad Wentlandt said a very experienced police officer "engaged an active shooter" outside the temple and "multiple rounds were exchanged".
He said the police officer was shot several times and is now being treated in hospital for multiple gunshot wounds.
President Barack Obama has been told about the fatal shooting spree at a Sikh temple in Wisconsin, a White House official has confirmed.
John Brennan, Obama's counterterrorism adviser, told him about the shooting.
The president is now receiving updates on the incident in suburban Milwaukee, the official said.
At least seven people have died, including a gunman, and at least three people are seriously wounded, local police and hospital officials said.
A shooting during a Sunday service at a Sikh temple has killed at least seven people, including the gunman, and injured "dozens" more, leaving at least three critically wounded.
Four people were shot inside the temple and three were shot outside.
Smiran Kaleka, a niece of one of the wounded men, said a number of gunmen walked into the kitchen of the temple in Oak Creek and started shooting. She said her mother had told her the shooters were white men.
Police in Milwaukee say they have killed an "active shooter" at the scene. A police spokesperson told the Associated Press that six people had been shot inside the temple.
Three men have been taken to hospital in Milwaukee after a shooting incident in a Sikh Temple.
Hospital spokeswoman Carolyn Bellin said one was in the operating room, another in a surgical intensive care unit and the third was being evaluated in the emergency room.
At least one gunman opened fire during morning services at a Sikh temple in Oak Creek, Wisconsin, and remained on the loose inside the temple, possibly holding hostages, according to witnesses.
Police, ambulances and the fire department were on the scene at the temple.
The dispatcher for Oak Creek Fire Department said first reports of the shooting came in about 10:30am local time.
Two witnesses said the shooter or shooters were still inside the building.