A law enforcement official told ABC News that the handgun was found in her Milwaukee apartment, and she was prohibited from possessing a gun as a convicted felon.
Meanwhile, more information is being released about Wade Michael Page, the man investigators say killed six people and critically wounded three others before being shot and killed by police.
Page posted frequent messages on Internet forums for skinheads, urging members to be more active for their cause.
"If you are wanting to meet people, get involved and become active," he wrote last year. "Stop hiding behind the computer or making excuses."
He played in white supremacist heavy metal bands with names such as Definite Hate and End Apathy, and described himself as a member of the "Hammerskins Nation," a skinhead group rooted in Texas that has branches in Australia and Canada, according to the SITE Monitoring Service, a Maryland-based private intelligence firm that searches the Internet for extremist activity.
The Hammerskins have been accused in multiple violent crimes, including murders, since 1980s.
Page had many tattoos covering his arms and torso, including one on his right arm below his shoulder with the number "838," which is a coded symbol indicating membership in the Hammerskins, according to Marilyn Mayo, Co-Director of the Anti-Defamation League's Center on Extremism.
He joined the military in Milwaukee in 1992 and was a repairman for the Hawk missile system before switching jobs to become an Army psychological operations specialist in a battalion at Fort Bragg, N.C.
Page was demoted in June 1998 for getting drunk on duty and going AWOL, two defense officials said, speaking on condition of anonymity. Page also received extra duty and was fined. He was discharged later that same year.
Suburban Milwaukee police had no contact with Page before Sunday, and his record gave no indication he was capable of such intense violence.
Federal officials said the gun used in the attack had been legally purchased. Page had been licensed to own weapons since at least 2008, when he paid $5 each for five pistol-purchase permits in North Carolina.
ABC News and the Associated Press contributed to this report.