Cook was taken into custody on the grounds that she is prohibited from having a gun as a convicted felon, a federal law enforcement official said. Cook was convicted in 2002 for fleeing and eluding a traffic officer.
Milwaukee police released a statement Tuesday after the arrest of Misty Cook, saying, "In a joint investigation with the FBI, the South Milwaukee Police Department has arrested Misty Cook on the crime of felon in possession of a firearm. Charges will be sought through the Milwaukee County District Attorney's Office."
There's no indication Cook was involved with the attack. Much like her ex-boyfriend, according to the Anti-Defamation League, Cook was involved in white supremacy. The ADL has photos of Cook wearing a T-shirt with the name of the extremist right-wing group Volksfront. The group said that Cook had not been involved since 2008.
University of Nebraska criminology professor Pete Simi didn't comment on Cook but says he interviewed Page for a project researching white supremacists from 2001-03.
"When he joined his first white power music band, this really changed his life," he said, adding that Page told him his neo-Nazi beliefs came after serving in the Army.
"He [Page] saw and experienced things that angered him, convinced him whites were victimized," Simi said.
Page drank excessively and never felt comfortable talking about his parent's divorce or his mother, who died when he was young, he added.
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Cook might be able to fill in the gaps of what transpired in the weeks and months prior to Sunday's shooting, sources said. Cook's relationship with Page fell apart in the weeks leading up to his alleged shooting spree.
"She had put his stuff in the basement and told him if he needed anything else just to get it out of the basement, she really didn't want him around," said Bernie O'Dea, Cook's mother.
Sharon and Terry Page, who said they are unrelated to the shooter, live below Cook. They said Cook and Wade moved in to the apartment March 1, but Wade moved out in the middle of June. After that point, according to comments Cook made to Sharon Page, Wade virtually disappeared.
"She told us that for six weeks she hadn't heard anything of him," Page said. "That would have been about the time after he left, so she must have gotten a hold of a co-worker and said he hadn't been at work for three weeks. She said he kind of just dropped off the face of the earth."
Sharon and Terry Page said Cook and Wade Page were quiet and "never really talked to us at all really."
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Meanwhile, in Oak Creek, Wis., thousands gathered Tuesday in an outpouring of support for victims of the Sikh temple shooting.
In a downtown park, attendees lit candles, flew orange Sikh flags, listened to Sikh music, joined in Sikh prayer, and donned patkas, a common Sikh head covering. Speakers described the victims' lives. Onstage a group formed a sign that read, "Practice Peace."
"What we learned, you know, about the shooter is he did stuff with music that was completely full of hate and energy that was brain-washing to himself," said Amardeep Kaleka.
Kaleka's father, Satwant Singh Kaleka, was president of the temple and among those shot and killed.
"It's still unreal. Unbelievable. Unfathomable," said Heather Owen, who attended the vigil with her two young children. "This is where I grew up. It's way too close to home."
But she expressed confidence that the town will rally in the face of tragedy.
"It's a tight-knit community and leaning on everybody's shoulders, that's what we do," she said.
Among the attendees were Gov. Scott Walker, Sen. Herb Kohl and Rep. Paul Ryan, who represents this district of Wisconsin.
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ABC News' Carol McKinley and Angela Hill contributed to this report.
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