The roughly 2,700 pages released by the Pima County Sheriff's Department were previously kept private.
The documents detail gunman Jared Lee Loughner's erratic and delusional behavior in the months leading up to the shooting rampage in Tucson, Ariz.
The files also provide a first glimpse into his family, showing that Loughner had grown nearly impossible to communicate with.
"I tried to talk to him. But you can't. He wouldn't let you," his father, Randy Loughner, told police. "Lost, lost and just didn't want to communicate with me no more."
"Sometimes you'd hear him in his room, like, having conversations," said his mother, Amy Loughner. "And sometimes he would look like he was having a conversation with someone right there, be talking to someone. I don't know how to explain it."
Randy Loughner said his 24-year-old son had never been diagnosed with mental illness. Despite recommendations from officials at Pima Community College, which expelled Loughner, that he undergo a mental evaluation, his parents never followed up.
According to the documents, however, his parents did drug test him. The results were negative, said his mother, who was worried her son might have been using methamphetamine. She said her son had told both parents he had not had a drink of alcohol in five months but he had tried marijuana and cocaine in the past.
Several weeks before the shooting, Loughner visited Anthony George Kuck, who had known him since preschool. Kuck said Loughner showed up to his house with a shaved head and a gun. Kuck said he kicked Loughner out of his house after he showed him the gun, which Loughner said was for protection.
"I tried to talk to him about why it's not smart to have a gun," Kuck said. "He obviously didn't listen to me."
On the day of the shooting, a friend, Bryce Tierney, told investigators Loughner called him early in the morning and left a cryptic voice mail that he believed was suicidal.
"He just said, 'Hey, this is Jared. Um, we had some good times together. Uh, see you later.' And that's it," Tierney said.
Another friend said Loughner grew increasingly isolated from people in the years leading up to the shooting.
The documents also contained witness accounts at the scene of the shooting. Giffords intern Daniel Hernandez described how constituents and others were lining up to see Giffords that morning. He helped people sign in and recalled handing the sheet on a clipboard to Loughner.
"The next thing I hear is someone yell, 'Gun,'" said Hernandez, who rushed to tend to Giffords' gunshot wound to the head.
"She couldn't open her eyes. I tried to get any responses from her. It looked like her left side was the only side that was still mobile," Hernandez told authorities. "She couldn't speak. It was mumbled. She was squeezing my hand."
Hernandez explained how he had some training as a nurse and first checked for a pulse. He said she was still breathing but her breath was shallow.
Loughner was sentenced in November to seven consecutive life sentences, plus 140 years, after he pleaded guilty to 19 federal charges.
His guilty plea enabled him to avoid the death sentence. He is serving his sentence at a federal prison medical facility in Springfield, Mo., where he was initially diagnosed with schizophrenia and forcibly given psychotropic drug treatments.
Arizona's chief federal judge and a 9-year-old girl were among the six people killed in the rampage. Giffords was left partially blind, with a paralyzed right arm and brain injury. She resigned from Congress last year and has since started, along with her husband, a gun-control advocacy group.
The Associated Press contributed to this report.