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Tucson, Arizona (CNN) -- The suspect in the weekend shooting at an Arizona political meet-and-greet is scheduled to make an initial appearance Monday before a federal magistrate.
Jared Lee Loughner will appear in a Phoenix, Arizona, courtroom at 2 p.m. Monday (4 p.m. ET), prosecutors said.
He is accused of opening fire outside a Tucson supermarket, where U.S. Rep. Gabrielle Giffords, D-Arizona, was meeting with constituents Saturday. Six people were killed and 14 others wounded in the shooting.
Giffords remained in critical condition Sunday after being shot through the skull, while a federal judge, a congressional aide and a 9-year-old girl were among the dead.
Federal authorities have charged Loughner with first-degree murder, attempted murder and attempting to kill a member of Congress, counts that involve the shootings of federal employees. State prosecutors also could bring charges in the remaining cases.
A federal public defender known for handling high-profile cases, Judy Clarke, has been appointed to represent Loughner, a federal judicial source said.
Clarke defended "Unabomber" Ted Kaczynski and assisted in the case of confessed al Qaeda operative Zacarias Moussaoui.
Loughner has invoked his right against self-incrimination and is not talking with investigators, Pima County Sheriff Clarence Dupnik said over the weekend.
But court documents released Sunday show that investigators found a letter from the congresswoman in a safe at the house where Loughner lived with his parents, thanking him for attending a similar 2007 event.
"Also recovered in the safe was an envelope with handwriting on the envelope stating, 'I planned ahead,' and 'my assassination' and the name 'Giffords,' along with what appears to be Loughner's signature," the affidavit says.
A law enforcement official said investigators are interviewing "anyone, everyone, we can." Loughner's parents were cooperative when interviewed, the official said.
Investigators have not found anything connecting Loughner to extremist groups, the law enforcement official said. They believe Loughner was unemployed at the time of the shooting, according to the official.
Months before Saturday's shooting rampage, one of Loughner's former instructors said he saw Loughner as a threat and kicked him out of class.
Loughner was "physically removed" from the Pima Community College algebra course in June -- less than a month after it began -- instructor Ben McGahee told CNN.
McGahee said Loughner sometimes shook, blurted things out in class and appeared to be under the influence of drugs at times, McGahee said.
"I was scared of what he could do," McGahee said. "I wasn't scared of him physically, but I was scared of him bringing a weapon to class."
The 9mm pistol used in Saturday's shootings was purchased at a gun store in November, FBI Director Robert Mueller told reporters Sunday. And a law enforcement source said the suspect tried to buy ammunition at a Walmart store but was turned down because of his behavior. Another Walmart store later sold him the ammunition, the source said.
When he tried to enlist in the Army in 2008, the service rejected him for reasons it says it can't disclose due to privacy laws. But an administration official told CNN on Sunday that Loughner had failed a drug test.
In postings on the social-media sites YouTube and MySpace, Loughner railed against government "mind control," being surrounded by people he considered illiterate and the illegitimacy of the U.S. government. In class, McGahee said Loughner accused him of violating his free-speech rights: "And of course free speech is limited in the classroom."
One such outburst was "the straw that broke the camel's back," and McGahee -- who had already raised concerns about Loughner with administrators -- had him removed.
Loughner "needed psychological help," and McGahee said he was not surprised to hear his former student was the suspect in Saturday's bloodbath.
"This guy was mentally disturbed. He was very isolated," he said.
In a statement Saturday night, Pima Community College said Loughner was suspended after a series of run-ins with campus police between February and September, capped by the discovery of a YouTube video in which he accused the college of operating unconstitutionally. Loughner quit school after the suspension, the college said -- and it warned him that to return, he had to present a doctor's note stating that his presence would not be "a danger to himself or others."
McGahee said the school responded to complaints about Loughner but "they didn't do it early enough."
"I think they did the best they can do, but as far as the time frame goes it could have been shortened," he said.
CNN's Jeanne Meserve, Bill Mears, Jessica Yellin and Barbara Starr contributed to this report.