By Peter Henderson and Tim Gaynor
TUCSON, Arizona (Reuters) - A 22-year-old man charged with trying to assassinate U.S. congresswoman Gabrielle Giffords in a shooting rampage that killed six people and wounded 14, is due to appear in court on Monday on charges of murder and attempted murder.
Doctors are optimistic that Giffords will recover after being shot in the head, but she is still in critical condition in a Tucson hospital following emergency brain surgery.
The shooting spree in Tucson on Saturday has fuelled debate about extreme political rhetoric in the United States after an acrimonious campaign for congressional elections in November.
The U.S. government has charged the suspected shooter, Jared Lee Loughner, with two counts of first-degree murder, one count of attempted assassination of a member of Congress and two other counts of attempted murder.
President Barack Obama called on Americans to observe a moment of silence on Monday at 11 a.m. EST (4 p.m. British time) to commemorate the victims of the shooting.
FBI Director Robert Mueller cautioned public officials to be on alert, but said there was no information to suggest a further specific threat.
Mueller said "hate speech and other inciteful speech" presented a challenge to law enforcement officials, especially when it resulted in "lone wolves" undertaking attacks.
"When the rhetoric about hatred, about mistrust of government, about paranoia of how government operates, and to try to inflame the public on a daily basis, 24 hours a day, seven days a week, has impact on people especially who are unbalanced personalities to begin with," said Clarence Dupnik, the sheriff of Pima County where the shootings occurred.
SUSPECT'S LAWYER DEFENDED UNABOMBER
Loughner was due to appear in court in Phoenix at 2 p.m. MST (9 p.m. British time).
He will be represented in court by Judy Clarke, the lawyer who defended Unabomber Ted Kaczynski, the federal defenders' office said.
Investigators said they had found an envelope at Loughner's 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 Glock pistol while the congresswoman was attending a political event in a supermarket parking lot. U.S. federal judge John Roll and a 9-year-old girl were among the six people killed.
Giffords, a 40-year-old Democrat, was in critical condition after surgery, but was able to follow simple commands, such as holding up two fingers when asked, doctors at University Medical Centre in Tucson said.
"I would say the best way of describing her this morning is that she is hold her own," Dr. Michael Lemole told CBS's "The Early Show" on Monday.
A single bullet travelled the length of her brain on the left side, hitting an area that controls speech. Given the devastating wound, doctors said they were uncertain about the extent of brain damage she may have suffered.
She 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," Lemole told reporters on Sunday. "I don't want to go down the speculation road but at the same time we're cautiously optimistic."
Dupnik said a wounded woman, identified as Patricia Maisch, had grabbed away an ammunition magazine from the gunman as he tried to reload after shooting into the crowd. He managed to fit in another magazine but it jammed and he was tackled by two men.
WASHINGTON SHOCKED
Gun violence is common in the United States but political shootings are rare and many in Washington were shocked.
Some Democrats said a 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.
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.
"I'm not aware of strong ties that he has to any group. This individual is a very troubled individual and he's a typical troubled individual who's a loner," Dupnik told ABC's "Good Morning America" program.
Loughner withdrew from Pima Community College in October 2010 after several encounters with campus police, college officials said. He was told to obtain a mental health clearance if he wished to return to school to show his attendance would not present a danger to himself or others.
The U.S. Army confirmed that Loughner attempted to enlist in December 2008 but was rejected for unspecified reasons.
Lawmakers in Washington put off their agenda for this week, including a vote on the repeal of Obama's contentious healthcare overhaul.
But several members of Congress appearing on morning talk shows on Monday said the tragedy would not stop them from holding public events.
"We cannot allow incidents like this to intimidate the public or intimidate us into continuing to be able to get up close and personal with our constituents," Democratic Representative Debbie Wasserman Schultz told CNN.
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.
Giffords had warned 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.
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.
(Additional reporting by Andy Sullivan, Richard Cowan, Tabassum Zakaria, Kim Dixon, JoAnne Allen and David Morgan in Washington, David Schwartz in Phoenix and Dan Whitcomb in Los Angeles; Writing by Steve Holland and Jeff Mason; Editing by Christopher Wilson and Eric Beech)
