as part of a joint investigation between the FBI and South Milwaukee Police Department "on the crime of felon in possession of a firearm."
A group called the Overpass Light Brigade hold a sign as they gather during a vigil at Cathedral Square to honour victims of Oak Creek in downtown Milwaukee, Wisconsin. Reuters/Allen Fredrickson
"Charges will be sought through the Milwaukee County District Attorney's Office," Milwaukee police said in a statement last night after the arrest of Cook, the former girlfriend of the slain 41-year-old gunman Wade Page.
A federal law enforcement official was quoted as saying by ABC News that Cook was taken into custody on the grounds that she is prohibited from having a gun as a convicted felon. Cook was charged in 2002 for fleeing and eluding a traffic officer.
Wade Michael Page is seen in this undated picture from a myspace.com web page for the musical group End Apathy. Reuters/myspace.com/Handout
Cook's relationship with Page fell apart in the weeks leading up to his alleged shooting spree at the gurdwara.
In Oak Creek, Wisconsin, thousands of people gathered last evening in an outpour of support for victims of the shooting on Sunday. They lit candles, flew orange Sikh flags and joined in Sikh prayers.
"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," Amardeep Kaleka, whose father Satwant Singh Kaleka was president of the gurdwara and among those shot and killed, was quoted as saying.
During the search operation at Cook's residence, police recovered a gun, but law enforcement officials said that the weapon was not involved in the shooting incident.
Cook had lived with Page at two separate apartments in South Miliwakee.
Jenna Benn, the Anti-Defamation League's assistant regional director in Chicago, told Stars and Stripes that Cook is affiliated with the white-power group Volksfront and is a supporter of the Hammerskin Nation, a Dallas-based white supremacy group.
She said the suspected gunman's ex-girlfriend is a prolific poster on hate forums.
Cook's neighbours told Fox news that Cook and Page separated recently.
One neighbour recently reported hearing Page "banging on the door of his old apartment, demanding to be let in."
David Brown, 62, a Navy veteran who also lived in the building, said Page often lifted weights in the basement, as angry music blasted away.
He said Cook began to change after living with Page.
"The longer she stayed here with him, the more quiet and more withdrawn she got," he was quoted as saying. "In the end she didn't even make eye contact with me."
According to the news channel, neighbours who lived in the building where Page and Cook shared a home for the first four months of 2012 said they were a strange couple, keeping to themselves.
When a Hispanic family moved into the building, the couple moved out.
"They had no stuff, and they didn't have movers, it was really weird," Robert Haugle, a resident of the building, said.
"Just one day they were gone," he told the news channel.
Mourners cry during a candlelight vigil at the Sikh Temple in Brookfield, Wisconsin. Reuters/John Gress