The Wall Street Journal / By ARIAN CAMPO-FLORES and CAROLINE PORTER

CR Note:  Sometimes confused for Muslims, Sikhism is a monotheistic religion founded in India in the 1400s...read on...

OAK CREEK, Wis.—A mass shooting at a Sikh temple south of Milwaukee left seven people dead Sunday, including the suspected gunman, in what police called an act of domestic terrorism. Three others were injured, including a police officer.

Police said the shooting began at about 10:30 a.m., as the congregation of the Sikh Temple of Wisconsin prepared for services. Chief John Edwards of the Oak Creek police said the suspect shot an officer multiple times before another officer shot and killed the suspect.

The injured officer was at a Milwaukee hospital, and Mr. Edwards said he was expected to recover. Names or ages of the suspect, the victims or the officers involved weren't released.

A federal law-enforcement official said evidence teams with the Federal Bureau of Investigation were called to the scene but that it was too early to tell exactly what was behind the shooting. The U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives had about a dozen agents on the scene Sunday, a second law-enforcement official said, and was tracing a firearm recovered at the temple site. The type of firearm couldn't be learned.

Earlier reports of multiple shooters at the temple were erroneous, police said.

The low-slung, one-story temple, which sits across from a bowling alley, was blocked off on Sunday afternoon, so family members and media gathered in a nearby parking lot cordoned off with yellow police tape.

Manjit Singh, a 30-year-old who attends the temple occasionally, said that on a typical Sunday about 200 people are at the service. He said Sunday mornings are usually spent in prayer, then end with a lunch at about 1 or 2 p.m. "If it had been later in the day, it could have been a lot more people," he said.

Lee Biblo, chief medical officer at Froedtert hospital in Milwaukee, said his hospital was treating three victims, all adult males, including the police officer. All were in critical condition: One was shot in the chest and abdomen; a second was shot in the neck; and a third was shot in the face and extremities. Two had undergone surgery as of late afternoon, he said.

The shooting comes a little more than two weeks since the movie-theater shooting in Aurora, Colo., that left 12 dead. The White House said Sunday afternoon that President Barack Obama had been briefed on the shooting by FBI Director Robert Mueller and other officials. Both the president and Republican presidential candidate Mitt Romney released statements, with Mr. Romney calling the shooting "a senseless act of violence and a tragedy that should never befall any house of worship."

Sikhism is a monotheistic religion founded in India in the 1400s, with estimates of the U.S. Sikh population ranging from 250,000 to more than 500,000. Sunday's shooting unnerved many Sikhs and triggered discussion of whether temples need to implement security measures, said Balwant Singh Hansra, past president of the Sikh Religious Society of Chicago. Because some Sikhs wear turbans, they are sometimes confused for Muslims, he said, and some Sikhs were targeted for violence or harassment in the aftermath of the Sept. 11, 2001, attacks.

The attack shocked many in this town of 35,000. Tom Thuecks, director of disaster services for the local branch of the Salvation Army, said he hadn't witnessed tension between different communities. "This community is a wonderful area, where people are concerned about others," he said. "All faiths, all denominations work together."

—Thomas Catan and Evan Perez contributed to this article.