(Reuters) - A 22-year-old man charged with trying to assassinate 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.  Giffords was still in critical condition at a Tucson hospital following emergency brain surgery. Doctors said on Monday it was a good sign that there was no increased swelling in her brain and that she continued to respond to simple commands such as squeezing a finger and wiggling her toes.

"Things are going very well," Dr. Peter Rhee, the hospital's trauma director told reporters. She will remain on ventilator for several more days, and is brought out of a medically induced coma several times a day to check her responsiveness, Dr Michael Lemole, chief neurosurgeon, said.

The shooting spree on Saturday in Arizona has fueled debate about extreme political rhetoric in the United States after an acrimonious campaign for congressional elections in November.

While the motive for the attack was not yet clear, several facts emerged about the suspect, Jared Lee Loughner, 22. People who knew him said he was a troubled young man who had been asked to leave a local college for disruptive behavior.

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.

President Barack Obama and first lady Michelle Obama bowed their heads in a moment of silence at 11 a.m. EST for the victims of the Arizona shooting.

About 300 White House staffers joined the Obamas on the South Lawn for the ceremony on a frigid day in Washington. Hundreds of people also gathered on the steps of the U.S. Capitol to also observe the moment of silence.

Flags there flew at half-staff in remembrance of a Giffords aide, Gabriel Zimmerman, who was killed in the shooting.

The U.S. government has charged 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.

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 (4 p.m. EST) on Monday, the Justice Department said.

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