The main suspect behind an attack on Arizona Congresswoman Gabrielle Giffords, which left her critically wounded and at least six people dead, has been named as 22-year-old Jared Loughner.
Witnesses said Mr Loughner opened fire with an automatic firearm in a supermarket where Ms Giffords was holding an event to meet her constituents. He was then wrestled to the ground.
Police have said he may have "a mental issue".
Pima County Sheriff Clarence Dupnik said the suspect "has kind of a troubled past".
The weapon has been identified as a Glock 19 9mm semi-automatic pistol which Mr Loughner purchased legally at a sports store in Tucson, Arizona, on 30 November, 2010.
Authorities say he had finished one magazine of bullets and was attempting to load a second when an unidentified woman ripped the gun away from him.
The woman was injured, and Mr Loughner tried to reload the gun but a spring mechanism failed - a mishap which potentially saved lives.
Mr Loughner has so far been charged with one count of attempted assassination of a member of Congress, two of first degree murder and two of attempted murder.
FBI Director Robert Mueller refused to comment on the gunman's motivation but he did not rule out the possibility of pressing additional charges under a domestic terrorism statute at a later date.
A classmate of Mr Loughner's told NBC News that he was very interested in politics and had met Ms Giffords and was unimpressed with her.
Sheriff Dupnik has confirmed that Mr Loughner attended a 2007 event with Ms Giffords. He was invited after corresponding with her staff members in her office.
The Washington Post reports that Mr Loughner registered as an independent voter in Arizona in the autumn of 2006, according to the Pima County Registrar of Voters.
Records indicate he voted in 2006 and 2008 but did not vote in last November's mid-term elections.
A search of Mr Loughner's home revealed a safe which held messages carrying the words "I planned ahead", "My assassination" and the word "Giffords", court documents said.
The FBI is also investigating a series of rambling messages posted on social networking sites in recent weeks.
Videos of text posted on a YouTube channel set up in the name of Jared Lee Loughner show a deep distrust of the government and religion, describing US laws as "treasonous" and speaking repeatedly of his wish to create a new currency.
"The government is implying mind control and brainwash on the people by controlling grammar," he wrote.
"No! I won't pay debt with a currency that's not backed by gold and silver! No! I won't trust in God!"
His internet postings also indicated an interest in hardline ideology, listing Karl Marx and Friedrich Engels's The Communist Manifesto and Adolf Hitler's Mein Kampf among his favourite books.
In a video dated 15 December, Mr Loughner describes himself as a "military recruit". The US Army has confirmed that Mr Loughner attempted to enlist but said he was rejected because he failed a drug test.
Hours before the attack on Ms Giffords, a message was posted on Mr Loughner's MySpace page saying "Goodbye".
"Dear friends, please don't be mad at me."
The page was immediately taken down once Mr Loughner's name was linked to the killings.
According to a memo leaked to Fox News, the department of homeland security is investigating Mr Loughner's possible ties to a white supremacist group called American Renaissance. The group's founder has denied that Mr Loughner was a member.
Mr Loughner lives close to the area where the shooting took place and had minor run-ins with the police for drugs-related offences, according to local reports.
He dropped out of school in 2006 before becoming a student at the Pima Community College. Officials there said he had been suspended in September after comments he made about the college online.
In October, he was asked to provide "a mental-health clearance" to show that he would not be a danger to himself or others if he returned to college.
The college said in a statement that he had five recorded run-ins with campus police over "classroom and library disruptions".
Schoolmates have described Mr Loughner as a loner who was often disruptive in classes.
"He was a guy in high school who definitely had his opinions on stuff and didn't seem to care what people thought of him," Grant Wiens told the Associated Press news agency.
"Sometimes religion was brought up or drugs. He smoked pot, I don't know how regularly. And he wasn't too keen on religion, from what I could tell."
In an e-mail dating from June and reprinted by The Washington Post, fellow student Lynda Sorenson described Mr Loughner as a "mentally unstable person" who could be "one of those whose picture you see on the news after he has come into class with an automatic weapon".
She said that she sat close to the door when he entered as Mr Loughner "scares the living crap out of me."
Gabriella Carillo said he had "caused a lot of trouble" in all his classes except for music. "If he tried, he would probably be at the top of our class. But he kind of just wasted his life. There are some guys who are just angry. I never really saw a smile on his face at all," she told AP.
A long-time friend of Mr Loughner, Tong Shan, told ABC News "he was a good person that just somehow changed so much".
Ms Shan had known Mr Loughner since high school and also had classes together at Pima Community College in 2007, but when she reconnected with him last year, she says he was very different.
"I don't know what might have caused him to change, but from the way he was talking to me [online]... you can see. It was just questions and questions and random, weird questions that didn't go together," she said. "He wanted to know everything... He would just trip out."
Caitie Parker, who knew him at high school, seemed to agree. "I think he slowly descended in a psychotic break. Something in him snapped. He wasn't always like this," she wrote on micro-blogging site Twitter.