"It was an attack not only against dedicated public servants but against citizens, one being a child," said FBI Director Robert Mueller during an afternoon press conference. "This was an attack on our institutions and an attack on our way of life."
Loughner is charged with one count of attempted assassination of member of Congress, two counts of killing an employee of the federal government and two counts of attempting to killing a federal employee.
Heather Williams, the first assistant federal public defender in Arizona, says the 22-year-old suspect doesn't yet have a lawyer, but that her office is working to get a lawyer appointed for Loughner.
Investigators said they seized evidence.
Investigators said they carried out a search warrant at the suspect's home and seized an envelope from a safe with messages such as "I planned ahead," "My assassination" and the name "Giffords" next to what appears to be the man's signature.
"These are preliminary charges and as the investigation goes on there will be additional charges that will be filed," Mueller said.
Loughner remains in federal custody and is being transported to Phoenix where he will make an initial court appearance at 2 p.m. on Monday.
Pima County Sheriff Clarence W. Dupnik shed more light on how the gun was secured during the shooting. A woman grabbed a magazine of ammunition away from the shooter. The shooter, according to Dupnik, was able to grab another magazine - but the spring in it failed. As the shooter's second magazine failed, two men were able to subdue the shooter until law enforcement arrived.
Authorities said Giffords, 40, was targeted at a public gathering by a man with a semiautomatic weapon around 10 a.m. Saturday outside a busy Tucson supermarket. Fourteen people were injured, including the congresswoman.
He also fired at her district director and shot indiscriminately at staffers and others standing in line to talk to the congresswoman, said Mark Kimble, a communications staffer for Giffords.
"He was not more than three or four feet from the congresswoman and the district director," Kimble said, describing the scene as "just complete chaos, people screaming, crying."
One of the victims was Christina-Taylor Green, 9, who was a member of the student council at her local school and went to the event because of her interest in government. She is the granddaughter of Dallas Green, the former manager of the Philadelphia Phillies major league baseball team.
She was born on 9/11 and featured in a book called "Faces of Hope" that chronicled one baby from each state born on the day terrorists killed nearly 3,000 people in the U.S.
The fact that Christina's life ended in tragedy was especially tragic to those who knew her. "Tragedy seems to have happened again," said the author of the book, Christine Naman. "In the form of this awful event."
The suspect's motivation was not immediately known, but Pima County Sheriff Clarence Dupnik described him as mentally unstable and possibly acting with an accomplice.
The sheriff's office sought a man photographed by a security camera near the scene, described as white with dark hair and 40 to 45 years old. Deputies said Sunday afternoon the man had come in for questioning and they determined.
The assassination attempt left Americans questioning whether divisive politics had pushed the suspect over the edge.
Giffords faced frequent backlash from the right over her support of the health care reform last year, and had her office vandalized the day the House of Representatives approved the landmark measure.
Dupnik lashed out at what he called an excessively "vitriolic" atmosphere in the months leading up to the rampage as he described the chaos of the day.
At a press conference on Sunday, Dupnik also addressed a lack of gun control in the state.
"I have never been a proponent of letting everybody in this state carry weapons under any circumstances that they want, and that's almost where we are," he said. "The legislature at this time is proposing that students and teachers be allowed to have weapons in schools and in college. You know, colleges ought to be run by the college presidents, not the Arizona legislature. But that's the ridiculous state to where we have become."
Dupnik said he did not know if the suspect had ever sought help for mental health issues or been committed.
"We have very, very serious problems in this community," Dupnik said. "Back in 1960, when I was a young cop on the beat, we put mentally ill people who were threats into a system that incarcerated them. Today they're out on the street, and we're paying a price for it."
The sheriff said the rampage ended only after two people tackled the gunman.
"He was definitely on a mission," according to event volunteer Alex Villec, a former Giffords intern.
A shaken President Barack Obama called the attack "a tragedy for our entire country."