The 22-year-old loner accused of trying to assassinate a US congresswoman and killing six others, appeared in court, his head shaved, a cut on his right temple and in handcuffs.
Jared Lee Loughner stared vacantly at a packed courtroom and sat down to listen to whispered instructions from his attorney.
Representative 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 injured in the gunfire outside an Arizona supermarket.
The shootings, which claimed the lives of six people, including a federal judge, a congressional aid and a nine-year-old girl, have dominated news in the US, prompting outrage and sparking debate over gun control and whether heated political rhetoric fuelled the incident.
Before the hearing began, Loughner's court-appointed attorney Judy Clarke whispered to the defendant, who only spoke to say "yes," when the judge asked if he understood that he could face life in prison -- or the death penalty for the killings. The judge ordered Loughner be held without bail.
Among the dead was nine-year-old Christina Taylor Green, who was born on the day of the September 11, 2001, terrorist attacks. Her funeral is on Thursday.
President Barack Obama will travel to Arizona tomorrow to attend a memorial service for the victims, a senior administration official said.
Earlier yesterday, Americans observed a moment of silence for the victims of the rampage, from the South Lawn of the White House and the steps of the US Capitol to legislature beyond Arizona and the International Space Station.
There, Ms Giffords' brother-in-law, Scott, the commanding officer, spoke over the radio. 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."