By the CNN Wire Staff
The suspect had railed against government "mind control" and illiteracy in online missives and had "kind of a troubled past," said Pima County Sheriff Clarence Dupnik.
"There's reason to believe this individual may have a mental issue," Dupnik said.
He said the suspect had previous contact with law enforcement in which he made violent threats, but he declined to provide further details. Court records indicate Loughner had been arrested for possession of drug paraphernalia in 2007, but the charges were dismissed.
Authorities said they did not know the motive for the shooting -- the suspect was not talking and had invoked his right against self-incrimination, Dupnik said.
Witness Dr. Steven Rayle said the shooter had a "determined look" as he opened fire.
"He was not sort of going around and picking out people and firing at them ... He was just firing his gun indiscriminately," he said.
The suspect was trying to reload his gun when he was tackled, said another witness, Joe Zamudio. Zamudio said he was one of the bystanders who pinned the gunman to the ground until police showed up.
"He was ready for war. He was not playing around," he said. "He was going to keep shooting. It was not over. He had just ran out of bullets."
Police recovered a 9mm Glock Model 19 handgun believed used in the shooting, according to senior law enforcement sources. The weapon had a extended magazine, according to a federal law enforcement source briefed on the investigation. The gun was purchased legally, a law enforcement source familiar with the investigation said.
In addition to the 12 people wounded, the shooting killed 63-year-old Chief Judge John Roll of the U.S. District Court for Arizona; 30-year-old Gabe Zimmerman, a Giffords staffer who was engaged to be married; 76-year-old Dorwin Stoddard, who was fatally shot in the head while trying to shield his wife; 76-year-old Dorothy Morris; and 79-year-old Phyllis Scheck.
The sixth victim, 9-year-old Christina Taylor Green, was pronounced dead at a hospital. She was born on September 11, 2001, according to CNN affiliate KVOA. She had just been elected to the student council at her school, the Arizona Republic reported.
House Speaker John Boehner said Sunday he had asked that flags in the House side of the Capitol be flown at half-staff, marking Zimmerman's death.
Dupnik has not stated a motive for the assassination attempt against the Democratic congresswoman. But he suggested that "vitriolic rhetoric" in political debates could have deadly consequences.
"When you look at unbalanced people, how they respond to the vitriol that comes out of certain mouths about tearing down the government, the anger, the hatred, the bigotry that goes on in this county is getting to be outrageous. Unfortunately, Arizona, I think, has become sort of the capital," he said. "We have become the mecca for prejudice and bigotry."
Giffords, 40, was shot once in the head at close range, authorities said. She underwent surgery at University Medical Center in Tucson and was one of five people listed in critical condition Sunday morning, hospital spokeswoman Darci Slaten said.
Four other victims taken to that hospital were in serious condition, Slaten said.
Dr. Peter Rhee of the University Medical Center said Saturday afternoon that he was "very optimistic" about Giffords' recovery after surgery, but noted that the next 24 hours would be key.
Giffords staffer Mark Kimble said the congresswoman did not have any security with her Saturday morning, which was not unusual for her. Her press secretary, C.J. Karamargin, said he was unaware of any recent threats against Giffords.
After the shooting, U.S. Capitol Police said members of Congress should take "reasonable and prudent precautions regarding their personal safety and security." And the House Sergeant at Arms also said it was "essential" that lawmakers contact local police to register their home and office addresses.
Tucson Police Chief Roberto Villasenor said his department had secured the homes and offices of some unnamed federal officials as a "precautionary measure."
All legislation on the House schedule for the coming week was postponed, House Majority Leader Eric Cantor said. The decision was made by leaders of both parties and means the House will not vote this week on the repeal of health care reform.
Rep. Chellie Pingree, D-Maine, said an incident like this could have a chilling effect on the frequent weekend listening post sessions many members of Congress have with their constituents.
"It does give you pause," Pingree said.
Zimmerman, the staffer who died, was in charge of community outreach in Giffords' Tucson office, Karamargin said.
Roll was a 1991 Bush appointee to the federal bench. The chief judge for the U.S. District Court for Arizona, he was originally from Pennsylvania.
Giffords was first elected in 2006. She has served as chairwoman of the House Space and Aeronautics Subcommittee and also holds seats on the House Science and Technology and Armed Services committees.
She won her third term in a closely contested race against a Tea Party-sponsored candidate and was one of three Democratic legislators who reported vandalism at their offices following the March vote on health care reform.
She is married to Navy Capt. Mark Kelly, a NASA astronaut who is scheduled to lead a space shuttle mission to the International Space Station. She is the only U.S. Representative with an active duty military spouse, according to her website.
In Tucson, supporters gathered outside the University Medical Center, keeping vigil as family members waited with wounded loved ones inside.
On the other side of the country, dozens of marchers braved sub-freezing temperatures and gusty winds to gather on Capitol Hill for a candlelight vigil Saturday night.
"I hope today will be the last day there will be any kind of violence against elected officials in this country," one woman said as the crowd formed a circle with the Capitol's dome in the background.
CNN's Jessica Yellin, Susan Candiotti, Jeanne Meserve, Dana Bash, Nick Valencia, Leslie Tripp, Steve Brusk and Traci Tamura contributed to this report.
