PHOENIX -- The 22-year-old loner accused of trying to assassinate a U.S. congresswoman and killing six others, appeared in court and looked on impassively as a judge told him he could face the death penalty for the shooting rampage that shocked the nation.

Rep. Gabrielle Giffords lay about a 100 miles away in an intensive care unit, gravely wounded after being shot through the head but able to give a thumbs-up sign that doctors found as a reason to hope.

Thirteen other people were wounded in the bursts of gunfire at the Democratic congresswoman's outdoor meeting with constituents Saturday outside a Tucson, Arizona, supermarket. Loughner was tackled to the ground minutes after the shooting began, authorities said. He has been silent ever since.

The shootings, which claimed the lives of six people, including a federal judge, a congressional aide and a nine-year-old girl, have prompted outrage throughout the U.S. and sparked a debate over gun control measures and whether toxic political rhetoric fueled the incident.

Jared Loughner, his head shaved, a cut on his right temple and in handcuffs, stared vacantly at the packed courtroom before sitting down to listen to whispered instructions from his newly appointed attorney, Judy Clarke. A veteran of death cases, the San Diego attorney succeeded in negotiating a guilty plea and a life sentence for the "Unabomber," Theodore Kaczynski.

Loughner seemed impassive and at one point stood at a lectern in his beige prison jumpsuit. A U.S. marshal stood guard nearby.

The judge asked if he understood that he could get life in prison -- or the death penalty -- for killing federal Judge John Roll, in the shooting rampage.

"Yes," he said. His lawyer stood beside him as the judge ordered Loughner held without bail.

Throngs of reporters and television news crews lined up outside the federal courthouse, where the hearing was moved from Tucson. The entire federal bench In Tucson recused itself because Roll was the chief judge there.

President Barack Obama will travel to Tucson on Wednesday to speak at a memorial service for the victims at the University of Arizona.

Earlier in the day, the nation observed a moment of silence for the victims, from the South Lawn of the White House and the steps of the U.S. Capitol to legislatures beyond Arizona and the planet itself. At the International Space Station, Giffords' brother-in-law, Scott Kelly, the commanding officer, spoke over the radio as flight controllers in Houston fell silent.

"As I look out the window, I see a very beautiful planet that seems very inviting and peaceful," he said. "Unfortunately, it is not.

"These days, we are constantly reminded of the unspeakable acts of violence and damage we can inflict upon one another, not just with our actions, but also with our irresponsible words," he said.

"We're better than this," he said. "We must do better."

On a frigid morning outside the White House, Obama and first lady Michelle Obama stood side by side, each with their hands clasped, heads bowed and eyes closed. On the steps of the U.S. Capitol, congressional staff and other employees did the same.

At the Supreme Court, the justices paused for a moment of silence between the two cases they were hearing Monday morning.

The president called for the country to come together in prayer or reflection for those killed and those fighting to recover.

"In the coming days, we're going to have a lot of time to reflect," he said. "Right now the main thing we're doing is to offer our thoughts and prayers to those who've been impacted, making sure we're joining together and pulling together as a country."

Later Monday, a moment of silence was held at the BCS national college football championship between Oregon and Auburn in Glendale, Arizona.

The shooting highlighted tensions that have been running high between conservatives and liberals in the United States, where activists and talk show radio hosts have been employing increasingly violent language in their criticisms of the Obama administration.

In 2009, a protester was discovered carrying a gun at a Giffords rally and there were signs the congresswoman was becoming concerned about the strident tone of the political debate in the U.S.

The day before Giffords was wounded, she sent an e-mail to a Republican friend discussing how to "tone our rhetoric and partisanship down."

In the message, obtained by The Associated Press, the Democratic congresswoman wrote to Republican Kentucky Secretary of State Trey Grayson to congratulate him on his new position at Harvard University.

"After you get settled, I would love to talk about what we can do to promote centrism and moderation," she wrote.

Giffords, narrowly managed reelection to a third term in 2010, in Arizona a conservative state known for its loose gun control laws and which made headlines last year after it passed a draconian anti-immigration law allowing police to stop those they suspected of being in the country illegally.

Giffords was a vocal opponent of the law and a supporter of Obama's healthcare law widely opposed by conservatives.

Loughner is charged 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. Those are federal charges.

State prosecutors, meanwhile, are researching whether they have to wait until after the federal case is resolved, or if they can proceed with local charges at the same time, an official said.

A Mass for all the victims was scheduled Tuesday at St. Odelia's Parish in Tucson.

Among the dead was 9-year-old Christina Taylor Green, who was born on the day of the Sept. 11, 2001, terrorist attacks. Her funeral is Thursday.

It was unclear when funerals will be held for the other victims, including one of Giffords' aides.

Giffords, 40, was shot in the head at close range. She was in critical condition at Tucson's University Medical Center. Two patients were discharged Sunday night. Seven others remained hospitalized.

Recent CT scans showed no further swelling in Giffords' brain, but doctors were guarded.

"We're not out of the woods yet," her neurosurgeon Dr. Michael Lemole said. "That swelling can sometimes take three days or five days to maximize. But every day that goes by and we don't see an increase, we're slightly more optimistic."

After Saturday's operation to temporarily remove half of her skull, doctors over the past two days had Giffords removed from her sedation and then asked basic commands such as: "Show me two fingers."

"When she did that, we were having a party in there," said Dr. Peter Rhee, adding that Giffords has also managed to give doctors a thumbs-up and has been reaching for her breathing tube, even while sedated.

"That's a purposeful movement. That's a great thing. She's always grabbing for the tube," he said.

Giffords' family is by her side and is receiving constant updates from doctors. Her doctors have declined to speculate on what specific disabilities Giffords may face.

With few new details emerging at the hearing, questions remained about what could have motivated someone to arm himself with a pistol and magazines carrying 33 bullets each, and rain gunfire on a supermarket parking lot crowded with men, women and children.

Comments from friends and former classmates bolstered by Loughner's own Internet postings have painted a picture of a social outcast with almost indecipherable beliefs steeped in mistrust and paranoia.