TUCSON, Ariz. (AP) — Authorities on Sunday charged a 22-year-old man described as a pot-smoking loner with trying to assassinate Rep. Gabrielle Giffords and killing others at a political event, revealing that he had scrawled on an envelope the words "my assassination" and "Giffords."

The discoveries at his home in southern Arizona, however, provided few answers to a shocked nation, from the victims of Saturday's shooting rampage to lawmakers worried about their safety: What motivated the rampage outside a supermarket that killed six and injured 14?

Giffords, 40, lay in intensive care at a Tucson hospital, after being shot in the head at close range. Doctors said she had responded repeatedly to commands to stick out her two fingers, giving them hope she may survive.

Court papers filed with the charges against Jared Loughner said he had previous contact with the Democratic lawmaker. The documents said he had received a letter from Giffords in which she thanked him for attending a "Congress on your Corner" event at a mall in Tucson in 2007.

Investigators carrying out a search warrant at his parents' home in a middle-class neighborhood found an envelope in a safe with the words "I planned ahead," ''My assassination" and the name "Giffords" next to what appears to be Loughner's signature.

An official familiar with the Arizona shooting investigation said Sunday that local authorities are looking at a possible connection between Loughner and an online group known for white supremacist, anti-immigrant rhetoric.

The official, who spoke on condition of anonymity to discuss the ongoing investigation, said local authorities were examining the American Renaissance website for possible motives.

The group's leaders said in a posting on their website that Loughner never subscribed to their magazine, registered for any of the group's conferences or visited their Internet site.

Authorities weren't saying Sunday where Loughner was being held, and officials were working to appoint an attorney for him ahead of a scheduled Monday afternoon court appearance in Phoenix.

Heather Williams, the first assistant federal public defender in Arizona, said they're asking that San Diego attorney Judy Clarke be appointed.

Clarke, a former federal public defender in San Diego and Spokane, Wash., served on teams that defended Oklahoma City bombing conspirator Timothy McVeigh, "Unabomber" Ted Kaczynski and Susan Smith, a South Carolina woman who drowned her two sons in 1994.

Loughner, who lived with his parents, was recently suspended from his community college for disruptive behavior and told he could not return until a mental health professional determined he was not a danger to himself or others.

Police say he purchased the Glock pistol used in the attack at Sportsman's Warehouse in Tucson in November. Authorities believe he acted alone: After questioning a cab driver who drove Loughner to the grocery store, they cleared the driver of any involvement.

Prosecutors charged Loughner with one count of attempted assassination of a member of Congress, two counts of killing an employee of the federal government and two counts of attempting to kill a federal employee. More charges are expected.

Clarke, a former federal public defender in San Diego and Spokane, Wash., served on teams that defended Oklahoma City bombing conspirator Timothy McVeigh, "Unabomber" Ted Kaczynski and Susan Smith, a South Carolina woman who drowned her two sons in 1994.

About 200 people, meanwhile, gathered outside Giffords' Tucson office Sunday evening for a candlelight vigil. Earlier in the day, people crammed the synagogue where Giffords has been a member, as well as the Mountain Avenue Church of Christ, which lost one member in the attack and saw another one wounded.

"I don't know how to grieve. This morning I don't have the magic pill, I don't have the Scripture... I can't wrap my head around this," said the church's Rev. Mike Nowak, his strong preacher's voice wavering.

Nowak said he received hundreds of e-mails from people sharing their prayers with the congregation.

Outside the hospital, candles flickered and people lay down bouquets of flowers, American flags and pictures of the personable politician they affectionately called "Gabby."

Giffords' colleagues, shocked at the violence, vowed not to let it deter them from meeting publicly, face to face, with their constituents. Some, however, acknowledged they were reviewing their security measures.

"I am very concerned about my safety and the safety of other members of Congress," said Rep. Bobby Rush, a Chicago Democrat. "I've informed those who are in my midst that they should be much more vigilant."

Rush said the climate is dangerous for political leaders, particularly for those who have supported President Barack Obama and his policies. He said he doesn't plan to scale back any public appearances.

"We need to realize that every face in the crowd is not a friendly face," he said.

Rep. Bill Huizenga, R-Mich., said he will proceed with public meetings. "I'm not going to let some lone gunman handcuff democracy," he said, adding he will take reasonable precautions.

Giffords, a conservative Democrat re-elected in November, faced threats and heckling over her support for immigration reform and the health care overhaul. Her office was vandalized the day the House approved the landmark health care measure.

It is not clear whether those issues motivated the shooter to fire on the crowd gathered to meet Giffords.

In a YouTube video, which featured text against a dark background, Loughner described inventing a new U.S. currency and complained about the illiteracy rate among people living in Giffords' congressional district in Arizona.

The six killed included a federal judge, John Roll; an aide to Giffords; and 9-year-old Christina Taylor Green, who was born on Sept. 11, 2001, and was featured in a book called "Faces of Hope" that chronicled one baby from each state born on the day terrorists killed nearly 3,000 people.

The author, Christine Naman, said: "Tragedy seems to have happened again."

Green was recently elected as a student council member and went to the morning's event because of her interest in government.

Fourteen others were injured, including the three-term Democrat lawmaker. Authorities said the dead included Roll; Green; Giffords aide Gabe Zimmerman, 30; Dorothy Morris, 76; Dorwin Stoddard, 76; and Phyllis Schneck, 79.