The mixed verdict is a blow for federal authorities, who spent years investigating the two men. Agents with the US Bureau of Alcohol, Tobacco, Firearms and Explosives finally had enough cause to arrest the brothers in 2009 but the investigation continued even after that, totaling eight years in all.
Despite the split result, the head of the ATF's Phoenix division, Thomas Atteberry, congratulated the investigators who worked on the extraordinary case, which led them to several states and also resulted in successful prosecutions of two other men associated with the brothers.
"Eight years...is a long time," Atteberry told TPM. "It goes to show you the tenacity and the dedication of the agents and the prosecutors that saw it to the end."
He said he was "disappointed" that Daniel Mahon would be set free, "but that does not take away from the outstanding work."
As TPM detailed in January, the case hinged on the use of a civilian woman who went undercover and befriended the men while federal investigators recorded their conversations on audio and video.
During the investigation, court records show, the brothers told the woman they had been involved in numerous bombings since the early 1980s. Dennis Mahon told her he had bombed an abortion clinic, a Jewish community center and offices of the IRS and immigration authorities.
Dennis Mahon was a former leader in the Ku Klux Klan. But more recently he and his brother were connected to the White Aryan Resistance, a loose association of white supremacists who encourage each other to live "lone wolf" lifestyles to avoid detection by law enforcement.
Even though the jurors convicted Dennis Mahon in the bombing, they determined the attack was not motivated by the race of the office's director, Don Logan, who is black.
Still, Acting US Attorney for Arizona Ann Birmingham Scheel saw the verdict as a win for racial justice. In a statement following the decision, she made no mention of the acquittal and instead focused on the conviction.
"This case shows that we will do all within our power to work with our law enforcement partners not only to prevent hate crimes, but also to vigorously prosecute those who commit them," she said.
Dennis Mahon is scheduled to be sentenced May 22. He faces up to 40 years in prison for the bombing.
ATF investigated the case with the help of the US Postal Inspection Service and the Scottsdale, Ariz. Police Department.