The gunman who killed six worshippers at a Sikh temple in Wisconsin was a convicted criminal who had been brought to the attention of federal authorities and monitored by civil rights groups years ago because of his ties to white supremacists.
Wade Michael Page, a skinhead who played in neo-Nazi rock bands, had been prosecuted for drunken driving and criminal mischief. But he was nonetheless able to legally purchase the 9mm handgun and ammunition used in Sunday's attack.
"He bought it legally; he was not an ineligible person," said James Santelle, the US attorney for the district which takes in Oak Creek, the town near Milwaukee where the mass shooting was carried out.
The Los Angeles Times reported that federal investigators repeatedly "looked at" claims that Page, 40, was helping to fund domestic terror groups in recent years.
Citing an unnamed "senior US law-enforcement official", the newspaper added that the authorities had been unable to find sufficient evidence of wrongdoing to open any formal investigation.
One officer remains in hospital after being among the three people seriously wounded. The attack is being treated as a hate crime and there are suspicions that he wrongly believed the victims to be Muslims.
The Springfield semi-automatic handgun used in the attack was bought at a local gun store called The Shooter Shop. Small, easily concealed and capable of firing large amounts of ammunition in a short time, is similar to the handguns used by both James Holmes, the perpetrator of the Aurora cinema shooting, and Jared Lee Loughner, who killed six people, and wounded Congresswoman Gabrielle Giffords, in Tuscon last year.
The FBI's Teresa Carlson told reporters that the agency was now looking into Page's "ties to the white supremacist movement". She added that investigators had no evidence to suggest that Page had any accomplices. "The definition of domestic terrorism is the use of violence for social or political gain. That's obviously what we are looking at."
Page is said to have shot a priest outside the temple. Evidence suggests he was then injured in a struggle inside with a second man, who later died. Police shot Page dead after that.