The moment was marked at the U.S. Capitol and elsewhere around a nation still coming to grips with the tragedy.
Giffords' brother-in-law, Scott Kelly, space station commander, led NASA in a moment of silence — and struggled with the senselessness of the shooting.
Flight controllers in Houston fell silent as Scott Kelly spoke via radio from space.
"We have a unique vantage point here aboard the International Space Station," he said. "As I look out the window, I see a very beautiful planet that seems very inviting and peaceful. 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. We must do better."
In Tucson, about a half dozen people gathered outside of Giffords' hospital during the moment of silence.
Prosecutors allege Loughner scrawled on an envelope the words "my assassination" and "Giffords" sometime before he took a cab to a shopping center where the congresswoman was meeting with constituents Saturday morning.
A federal judge, a congressional aide and a 9-year-old girl, Christina Taylor Green, were among the six people killed, while Giffords and 13 others were injured in the bursts of gunfire outside a Tucson supermarket.
At Christina's Mesa Verde Elementary School Monday morning, a memorial of ribbons across a fence in front of the school was slowly growing as students arrived. Flowers, candles and cards signed by classmates and students at other schools also surrounded the fence.
Associate Superintendent Todd Jaeger said Monday that teachers would meet with a team of psychologists to discuss how to talk to kids about the third grader's death.
"One of the things we know is that we have to be honest with kids and answer their questions. We need to answer those questions without adding to their angst," Jaeger said.
At a Tucson hospital, Giffords, 40, remained in intensive care Monday after being shot in the head at close range.
Neurosurgeon Dr. Michael LeMole of Tucson's University Medical Center, said her condition is stable. The swelling in Giffords' brain has not been increasing, as typically occurs during the first three days of such an injury.
"That's why we are much more optimistic and we can breathe a collective sigh of relief after about the third day," he told reporters.
He said there are other good signs. The track of bullet is away from the nerves. "Not only are those centers of the brain working but they're communicating with one another."
LeMole said Giffords is still responding to commands to squeeze hands, move her toes, etc.
Of those injured in the deadly shooting Saturday in Tucson, eight are still hospitalized. One is in critical condition, five are in serious condition, and two are in good condition.
A federal judge, a congressional aide and a 9-year-old girl, Christina Taylor Green, were among the six people killed, while Giffords and 13 others were injured.
Authorities weren't saying where Loughner was being held, and officials were working to appoint an attorney for him.
The federal public defender in Arizona has called on San Diego attorney Judy Clarke, a former federal public defender who served on teams that defended McVeigh, a coconspirator in the 1995 Oklahoma bombing and other high profile cases.
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. More charges are expected.
Discoveries at Loughner's home in southern Arizona, where he lived with his parents in a middle-class neighborhood have provided few answers to what motivated him.
Police say he has not been cooperating with investigators.