A 22-year-old man will appear in court on Monday charged with attempting to assassinate a US congresswoman during a shooting spree in Arizona in which six others died, prosecutors said.
President Barack Obama called on Americans to observe a moment of silence on Monday for victims of the attack as some officials asked if the nation's often divisive politics had spiraled out of control.
The lone gunman, who was overpowered by bystanders at the scene of the shooting, will appear in a federal court in the state capital Phoenix at 2:00 pm (2100 GMT) charged with five counts including murder and attempted murder.
Representative Gabrielle Giffords, 40, was in an induced coma as doctors treated bullet wounds to her head. Outside her Tucson hospital, well-wishers left candles, flowers and notes including one that read, "Fight, Gaby, Fight!"
Authorities said Jared Loughner, a 22-year-old booted last year from a community college, fired 31 shots as Giffords met with constituents at a grocery store in Tucson on Saturday.
He was overpowered as he tried to reload his 9mm Glock semi-automatic pistol, which he bought at a local store.
"As the investigation goes on, there may well be additional charges that will be filed," Robert Mueller, director of the Federal Bureau of Investigation, told reporters in Tucson.
Mueller said that no "specific threat" remained, although authorities urged lawmakers to be careful. Police questioned a taxi driver seen entering the Safeway store with Loughner but concluded he was only seeking change for the fare, sheriff's deputy Jason Ogan said.
Prosecutor said Loughner went to a similar public meeting with Giffords in August 2007. Investigators searched a safe at Loughner's home, where he lived with his family, and found a letter from Giffords thanking him for his attendance, the criminal complaint said.
Also in the safe they found an envelope with the hand-written notes, "I planned ahead," "My assassination" and "Giffords," it said.
Officials declined to assess Loughner's motives or mental state. He wrote a stream of barely coherent postings on the Internet that showed an interest in developing a new currency and criticism of "illiterate" fellow residents.
Pima Community College, which Loughner had attended, said he had been suspended for conduct violations and withdrew in October after five instances of classroom or library disruptions that involved campus police, according to The New York Times.
But Pima County Sheriff Clarence Dupnik criticized the political climate in Arizona, which shot to the international spotlight last year over tough laws cracking down on illegal immigration from Mexico, an hour south of Tucson.
"The rhetoric about hatred, about mistrust of government, about paranoia of how government operates ... has an impact on people, especially who are unbalanced personalities to begin with," said Dupnik, a member of Obama's Democratic Party.
Giffords, who narrowly won re-election last year over a favorite of the conservative Tea Party movement, is a centrist Democrat who supports increased border security and, incidentally, loose restrictions on gun ownership.
Lawmakers of the rival Republican Party, which swept last year's election, denounced the attack and suspended proceedings of the House of Representatives whose new leadership had taken over just three days earlier.
John Boehner, the new house speaker, said in his Ohio constituency that an "attack on one who serves is an attack on all who serve."
"No act, no matter how heinous, must be allowed to stop us from our duty," he said.
Doctors said it was too early to say how long Giffords' recovery would take. Many patients with such serious wounds need at least months to return to anything close to normal life.
But the doctors voiced optimism after her rapid progress, helped in part by the fact that the bullet did not go through both hemispheres of her brain.
"Because of that, Congresswoman Giffords is able to communicate with us... following simple commands. We're very encouraged by that," said Michael Lemole, the neurosurgeon who operated on her at University Medical Center.
"Brain swelling at any time can take a turn for the worse. But I am cautiously optimistic," he told reporters.
Authorities hailed the courage of Patricia Maisch, who was waiting to have her photograph taken with the congresswoman when Loughner opened fire. Maisch grabbed the bottom of the suspect's weapon to prevent him from reloading.
"This is one of the most heroic acts I've ever seem," Dupnik, the sheriff, told Fox News.
But six people were killed and at least 14 others were injured. Among the dead was federal judge John Roll, who came to speak with the congresswoman, and a nine-year-old girl.
The girl, Christina Taylor Green, was born on September 11, 2001. Her parents said her accident of birth gave her a precocious interest in politics, explaining why she went to Giffords' public event.
But her father John Green, fighting back tears, told CNN: "It does say something about our society that my daughter was born on a tragic day, and she went out on a tragic day."