An Arizona judge has ordered the 22-year-old man accused of killing six people and injuring 14, including a U.S. congresswoman, to be held without bail as he awaits trial on federal charges related to the shootings.

Jared Loughner entered the courtroom shortly after 2 p.m. local time (4 p.m. ET), handcuffed and wearing an inmate uniform.

His expression was impassive as he walked in, looked straight at the crowd at the back of the room packed with reporters, then turned around to speak to his lawyer, Judy Clarke. He responded "yes" when asked if he understood his rights.

The courtroom was under heavy guard with about a dozen U.S. marshals.

Clarke, a public defender, is known for her defence of "Unabomber" Ted Kaczynski and Sept. 11 conspirator Zacarias Moussaoui.

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.

Pima County Sheriff Clarence Dupnik said state charges against Loughner will also be filed by the county attorney's office on recommendations from his office. He said there is no rush to lay the additional charges because Loughner is in custody.

Police say he has not been co-operating with investigators. If convicted, the college dropout could face the death penalty, though prosecutors have not indicated whether they will pursue it.

Doctors treating Democratic Representative Gabrielle Giffords said she remained in critical but stable condition.

"At this phase in the game, no change is good," said Dr. Michael LeMole, Gifford's primary physician. "And we have no change."

In a briefing at University Medical Center in Tucson, LeMole said Giffords is on a ventilator and responding to simple commands. No more swelling in her brain has been observed and she remains in a medically induced coma.

LeMole added that she is not "out of the woods yet."

"That swelling can take three days to five days to maximize," he said. "Every day that goes by and we don’t see an increase we are slightly more optimistic."

Moment of silence

Earlier on Monday, President Barack Obama and his wife, Michelle Obama, led a national minute of silence at the White House for the victims of the shooting.

Representative Gabrielle Giffords, in a March 2010 photo. Giffords was critically wounded during a shooting at a political event Saturday in Tucson, Ariz. (Office of Rep. Gabrielle Giffords/Associated Press)

The moment of silence was also observed at the U.S. Capitol building and on the trading floor of the New York Stock Exchange.

Later, Obama called the shooting a "heinous crime" and "mindless violence."

"I think it’s important for us to also focus, though, on the extraordinary courage that was shown during the course of these events," Obama told reporters. "Part of what I think that speaks to is the best of America, even in the face of such mindless violence."

The president will fly to Tucson on Wednesday to attend a memorial service for the victims, two senior administration officials said late Monday.

The shooting took place at a public constituents meeting organized by Giffords at a Tucson supermarket.

Daniel Hernandez, an intern for Giffords, was standing about 15 metres away from the congresswoman, directing the public toward her, when the shooting began. He rushed to Giffords's aid after she was hit.

"The first thing I did was actually pick her up and kind of cradle her up against my body, up against my chest, to make sure she was sitting upright so she could breathe," he told CBC News. "Once I had her breathing OK, I then started applying pressure to her wound to try and stem the blood loss as much as possible."

Hernandez said he then tried to keep Giffords calm, telling her he would contact her family.

The dead include a federal judge, a congressional aide and a nine-year-old girl.

Dupnik said on ABC's Good Morning America on Monday that Loughner had said "not a word" to investigators. He said officials are almost certain the gunman acted alone, adding "[Loughner's] a typical troubled individual who's a loner."

Investigators are still trying to determine a motive for the shootings. They said that in a search of the home of Loughner's parents in southern Arizona, they found an envelope in a safe with "I planned ahead," "My assassination" and "Giffords" written on it, along with what appeared to be the signature of the accused.

A U.S. army official confirmed that Loughner was rejected by the military in 2008 for failing a drug test. The official, who spoke on condition of anonymity, did not disclose the type of drug.

The weekend shooting is having an effect on the political business of the United States. John Boehner, the Speaker of the House of Representatives, said normal House business this week has been postponed in the aftermath of the shooting.

Giffords, a conservative Democrat, narrowly won re-election in November 2010.

With files from The Associated Press