Sikh Temple of Wisconsin in Oak Creek
Members of the Sikh Temple of Wisconsin in Oak Creek wait for news following a mass shooting inside and outside the temple. Reuters

Hours after a lone gunman opened fire at the Sikh Temple of Wisconsin in Oak Creek on Sunday, federal investigators are considering the attack to be a case of domestic terrorism, based on the triggerman's background and the tattoos on his body.

The gunman killed six people and critically wounded three others during the rampage in the Milwaukee suburb, according to Reuters.

Law-enforcement officials have not identified the triggerman by name, but Thomas Ahern, a representative of the Chicago Field Division of the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, told CNN he was a white male about 40 years of age.

The gunman was killed in a shootout with police officers, one of whom is among the critically wounded.

Because of the ongoing investigation, officials are reluctant to publicly disclose details about the shooter and his background. However, the FBI -- the lead agency in the case -- did say that, based on certain evidence, the shooting appears to be domestic terrorism. Officials did not say whether they had concluded the man was conclusively linked to a hate group.

"The investigation will have to continue to see and determine the motive," a federal law-enforcement official told the Los Angeles Times. "We don't know much about the motive at this point."

Meanwhile, FBI Milwaukee Special Agent in Charge Teresa Carlson said in a statement: "The FBI is working closely with the Oak Creek Police Department and other local and federal agencies to investigate today's shooting incident. 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."

The FBI defines terrorism as "the unlawful use of force or violence against persons or property to intimidate or coerce a government, the civilian population, or any segment thereof, in furtherance of political or social objectives."

While the mass shootings in Aurora, Colo., last month were not labeled terrorism because there was insignificant evidence of political or social motivation, the mass shootings in Oak Creek, Wis., on Sunday may have been labeled as such due to the fact there may be significant evidence of that kind of motivation.