A white supremacist, who killed six Sikhs at a gurudwara in the US state of Wisconsin in August, had acted alone and there was no evidence that the attack was part of any ongoing threat to the community, the FBI has said, concluding its probe into the tragic incident.

"The FBI investigation indicates Wade Michael Page acted alone and was not assisted in committing this violent crime killing six and wounding four other victims. No evidence was uncovered to conclude this attack was directed or facilitated by any white supremacist group," Teresa Carlson, Special Agent in charge of the FBI Milwaukee Division, said.

The FBI briefed the victims and family members of the deceased on the results of the investigation on November 16, it said.

"We join the Sikh community in grieving the loss of their loved ones. We continue to work with temple leaders and all of our law enforcement partners in an effort to keep the community safe," Carlson said.

During the shooting at the gurudwara, 40-year-old Page "exchanged gun fire with two Oak Creek police officers seriously wounding one, before being shot by another officer, then turning his weapon on himself. There is also no evidence to suggest the attack was part of any ongoing threat to the Sikh community," the FBI statement said.

During the course of its investigation, the FBI generated 200 investigative leads; conducted 300 interviews; and collected more than 200 pieces of evidence.

Thirty agencies responded to the incident, including 27 local, one state and two federal agencies, Carlson said.

Meanwhile, a toxicology report from the Milwaukee County said Page had no drugs in his system at the time of the rampage.

Page did have a small amount of alcohol in his system, the report said. However, the medical examiner's office says alcohol is sometimes a by-product of decomposition. It's unclear whether Page had been drinking before the attack.

Members of the Sikh community had been briefed in advance of the FBI's report, Milwaukee Journal Sentinel said.

Page had moved to Milwaukee in late 2011, perhaps for a girlfriend who might have shared his white-power beliefs. But Page and Misty Cook broke up weeks before the shooting.

Page's strong beliefs in the white power movement had led to speculation that an organised group of white supremacists had been involved in the temple shootings. But the FBI report concludes that there was no evidence to make that connection, the report said.