The start of the legal process came as doctors gave an upbeat prognosis about the recovery prospects for Giffords, who is in critical condition after taking a bullet to the head Saturday.
President Barack Obama led a national moment of silence Monday to pay tribute to the victims. The suspected shooter, Jared Lee Loughner, 22, is charged with shooting Giffords outside a supermarket in Tucson, Arizona. He is suspected of killing six people and wounding 14 who had gathered to meet Giffords at a constituent event.
He faces five federal charges stemming from the attack, including two murder charges for the deaths of a federal judge and one of Gifford's aides, and three attempted murder charges for the assault on Giffords and two other federal officials.
He will be charged separately in Arizona state court for the attacks on the other victims targeted at the political event. A nervous-looking Loughner appeared in court in handcuffs and a prison uniform for about 15 minutes, answering the judge's questions with short, polite answers, CNN reported.
The dead included a nine-year-old girl who had just been voted to her school student council and wanted to speak to a real-life elected official. The attack triggered public discussion whether the strident tone of US political debate, as seen with the rise of the conservative Tea Party movement, influenced the shooter.
Pima County Sheriff Clarence Dupnik, who is overseeing the investigation of the mass shooting, lashed out at the national atmosphere and in his home state of Arizona, which is on the frontline of the debate over illegal immigration.
"I think when the rhetoric about hatred, about mistrust of government, about paranoia about how government operates ... and the (inflammation) of the public on a daily basis, 24 hours a day, seven days a week, has impact on people who are unbalanced personalities to begin with," Dupnik said in broadcast remarks.
The alleged shooter Loughner reportedly acted alone, after police dropped pursuit of a second lead. Loughner's alleged motives remain unclear. Media outlets have unearthed anti-government ramblings he has posted online, as well as sites where he expressed an admiration for books like Adolf Hitler's Mein Kampf.
Doctors said Monday that Giffords was in critical but stable condition after they had removed part of her skull to accomodate swelling of the brain. Michael Lemole said she had responded positively to questions after surgery and to instructions from her medical team.
"Things have stabilized and are looking better," he said at a press conference. "We're not out of the woods yet" but are optimistic about her prospects of recovery.
Giffords was the only one of the injured still listed in critical condition. Police have said the shooter was intentionally targeting Giffords, a moderate, three-term Democrat. Giffords came in for intense criticism in her conservative home state after she voted for Obama's health care reform bill. Her district offices had been vandalized and the door broken down, reports said.
Earlier this year, Sarah Palin, the darling of the Tea Party movement, used her Facebook page to post cross hairs over Gifford's district along with 19 other seats to be knocked out for supporting health reform. "Don't Retreat, instead - RELOAD!" she told supporters.
Giffords warned at the time that "when people do that, they have to realize there are consequences to such actions," according to past video of her remarks aired Sunday.