Though police have not given any details on the identity or motive of the shooter, or released the identities of the victims, sources have told ABC News the shootings are the work of a "white supremacist" or "skinhead."
Bureau of Alcohol, Tobacco, Firearms Special Agent Thomas Ahern said the suspect had tattoos and that authorities were investigating whether he was a "skin head or "white supremacist" as two ABC Sources said.
"It is being investigated. And what his tattoos signified is being investigated. They are all pieces of a possible puzzle to learn what was his motive in carrying out such a horrific act," Ahern said.
This evening, the FBI and a bomb squad arrived at a home in Cudahy, Wis., near Oak Creek, and ABC News Milwaukee affiliate WISN reported the action appeared to be related to the temple shootings earlier in the day.
Authorities also were trying to trace a single, semiautomatic handgun recovered at the scene, sources told ABC News.
In addition to the seven confirmed dead, three people -- two adult male civilians and a male police officer -- were in critical condition and were being treated at a local hospital, said officials at Froedtert & The Medical College of Wisconsin.
The apparent gunman was shot and killed in an exchange of gunfire with the wounded police officer outside the temple and was one of the seven dead.
"The officer stopped a tragic event that could've been a lot worse," Oak Creek Police Chief John Edwards told reporters.
Four people were found dead inside the temple and two others were found dead outside the building.
Edwards said authorities were treating the event as a domestic terrorism incident and the FBI would be conducting a full investigation.
"The FBI is working closely with the Oak Creek Police Department and other local and federal agencies to investigate today's shooting incident," FBI Milwaukee Special Agent in Charge Teresa Carlson said in a written statement. "This remains an active investigation in its early stages. While the FBI is investigating whether this matter might be an act of domestic terrorism, no motive has been determined at this time. We know our community has been deeply impacted by this incident, and our thoughts are with those affected and particularly with the officer who was wounded in the line of duty to protect others."
Individuals attending Sunday services at the Sikh Temple of Wisconsin, just south of Milwaukee, fled in all directions this morning when a gunman entered and began firing. Many hid in bathrooms or other rooms within the temple while the shooter attacked, according to police.
The president of the temple, Satwant Singh Kaleka, was preparing to deliver remarks when he became one of the shooting victims. His son, Amardeep Kaleka, spoke by phone with ABC News' David Muir shortly after getting a call from the priest using his father's phone.
"I picked it up immediately thinking it was my dad, but it was the priest and he was standing right next to him," Kaleka said. "He told me right away that right now my father can't speak. There's too much blood coming out of his back area and we have to get ambulances in there right away."
Soon, he heard briefly from his mother, also in hiding in the temple and asking for information about his father.
Edwards said 911 calls began pouring into the police department around 10:25 a.m.
The first police officer to respond to the scene, a 20-year veteran on the police force, exchanged gunfire with the suspect and sustained multiple gunshot wounds. He was undergoing surgery at Froedtert Hospital, the main trauma center in the Milwaukee region, along with two other injured victims.
Edwards said that by 5:30 p.m. EST today the temple had finally been cleared of all remaining people and checked for additional threats.
Initial reports of the shooting this morning included reports of additional gunmen, though authorities now believe the reports all described the same gunman.
According to information broadcast over police radio, a witness to the shooting told law enforcement the shooter was a white male, bald, with a heavy build. He was wearing a sleeveless T-shirt, according to Oak Creek Patch.
Biographical details and tattoos on the body of the slain gunman led officials to make the domestic terrorism designation, according to a report in the Los Angeles Times.
Police tactical teams spent more than four hours securing the temple and, at one point, police asked media outlets to stop broadcasting aerial footage from helicopters on television because of tactical operations at the scene.