By Tim Gaynor and Peter Henderson
TUCSON, Arizona (Reuters) - The government charged a 22-year-old man with attempted assassination of a congresswoman on Sunday while doctors expressed optimism that the wounded lawmaker, Gabrielle Giffords, would recover.
The shootings of Giffords and 19 other people -- six of whom were killed -- in Tucson on Saturday fueled debate about extreme political rhetoric in the United States after an acrimonious campaign for congressional elections in November.
The government charged Jared Lee Loughner with two counts of first degree murder, one count of attempted assassination and two other counts of attempted murder. He was due to appear in court in Phoenix on Monday afternoon, the Justice Department said.
Investigators said in the charges they found an envelope at his residence with the handwritten phrases "I planned ahead" and "My assassination," along with the name "Giffords" and what appeared to be Loughner's signature.
The suspect opened fire with a semi-automatic pistol at point-blank range outside a supermarket. U.S. federal judge John Roll and a 9-year-old girl were among the six people killed. Fourteen people were wounded.
The FBI said it had cleared a man officials had earlier sought to locate in connection with the shootings.
"At this time we don't have any reason to believe that there were any other individuals" associated with Loughner in the shooting, said special agent and FBI spokesman Manuel Johnson.
Public officials should be on alert but there was no information to suggest a specific threat, FBI Director Robert Mueller told a news conference.
But Mueller said "hate speech and other inciteful speech" presented a challenge to law enforcement officials, especially when it resulted in "lone wolves" undertaking attacks.
President Barack Obama called on Americans to hold a moment of silence on Monday at 11 a.m. EST to commemorate the victims of the shooting.
Giffords, a 40-year-old Democrat, was in critical condition but was able to follow simple commands, such as holding up two fingers when asked, doctors at University Medical Center in Tucson said.
A single bullet traveled the length of her brain on the left side, hitting an area that controls speech functions. Given the devastating wound, doctors said they were uncertain about the extent of brain damage she had suffered.
Giffords has been put into a pharmaceutical coma but was being awakened frequently to check her progress.
"There are obvious areas of our brain that are less tolerant to intrusion," said Dr. Michael Lemole. "I don't want to go down the speculation road but at the same time we're cautiously optimistic."
Gun violence is common in the United States but political shootings are rare.
Pima County Sheriff Clarence Dupnik said a wounded woman clawed away an ammunition magazine from the gunman, possibly preventing even more people from being shot. The gunman managed to reload his Glock pistol but the fresh magazine would not work and the suspect was then tackled by two men.
The violence shocked politicians in Washington. Some Democrats were quick to say a shrill climate of political vitriol might have played a role.
"We are in a dark place in this country right now and the atmospheric condition is toxic," Democratic Representative Emanuel Cleaver told NBC's "Meet the Press" program.
But Jon Kyl, a Republican senator from Arizona, cautioned against a "rush to speculate."
"We really don't know what motivated this young person, except to know he was very mentally unstable," Kyl said on the "Face the Nation" show on CBS.
Investigators were looking at a rambling Internet manifesto left by Loughner or someone writing under that name. There was no coherent theme to the writing, which accused the government of mind control and demanded a new currency.
The U.S. Army confirmed the suspect attempted to enlist in December 2008 but was rejected for unspecified reasons.
Obama signed a proclamation calling for flags to be flown at half-staff. House of Representatives Speaker John Boehner said the incident was a reminder that public service comes with a risk.
Lawmakers in Washington put off their agenda for this week, including a vote on the repeal of Obama's contentious healthcare overhaul. The president postponed a visit on Tuesday to a division of General Electric in Schenectady, New York.
The new Congress convened last week after the November 2 elections in which the Republican Party won control of the House and reduced the Democratic majority in the Senate.
The U.S. Capitol Police cautioned members of Congress "to take reasonable and prudent precautions." Still, most lawmakers are largely unguarded outside the Capitol, except the leaders of the House and Senate, who have security details.
Giffords warned previously that the heated rhetoric had prompted violent threats against her and vandalism at her office. Mueller said the suspect had attended a public event held by Giffords in 2007.
In an interview last year with MSNBC, Giffords cited a map of electoral targets put out by Sarah Palin, a Republican former Alaska governor and prominent conservative, that had each marked by the crosshairs of a rifle sight.
After the shooting, the graphic was removed from Palin's website and she offered condolences on a posting on Facebook.
A Palin aide, Rebecca Mansour, told conservative radio host Tammy Bruce of the graphic: "We never, ever, ever intended it to be gun sights. It was simply crosshairs like you'd see on maps ... a surveyor's symbol."
In several videos on the Internet site YouTube, a person with the name Jared Lee Loughner criticizes the government and religion. It was not known whether he was the same person as the suspect.
"The government is implying mind control and brainwash on the people by controlling grammar," the man says. "No! I won't pay debt with a currency that's not backed by gold and silver! No! I won't trust in God!"
Giffords, married to NASA astronaut Navy Captain Mark Kelly, is seen as a rising star in the Democratic Party. She narrowly defeated a conservative opponent and was one of the few Democrats to survive the Republican sweep in swing districts in November's elections.
Arizona has been at the center of a political firestorm in the past year, symbolizing a bitter partisan divide across much of America.
The spark was the border state's move to crack down on illegal immigration last summer, a bill proposed by conservative lawmakers and signed by the state's Republican governor, Jan Brewer.
Giffords had said the move would not secure the border or stop drug smuggling and gun running.
(Additional reporting by Andy Sullivan, Richard Cowan, Tabassum Zakaria and Kim Dixon in Washington, David Schwartz in Phoenix and Dan Whitcomb in Los Angeles; Writing by Steve Holland and Jeff Mason; Editing by David Storey)