Jared L. Loughner, identified by US federal law enforcement officials as the suspected gunman in the shooting of US Representative Gabrielle Giffords and others in Tucson on Saturday, is seen in this undated high school yearbook photo from the 2006 Mountain View High School yearbook obtained from a classmate of Loughner's in Tucson, Arizona on Jan 8, 2011. [Photo/Agencies]

PHOENIX, Arizona - A troubled 22-year-old college dropout made his first court appearance on Monday on five federal charges, including the attempted assassination of US Representative Gabrielle Giffords, who remained in critical condition.

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His hands manacled together, Jared Lee Loughner leaned toward a microphone and told a judge in Phoenix he understood the charges against him that stem from a shooting spree in Tucson on Saturday that left six people dead and 14 wounded.

Loughner walked into the courtroom looking straight ahead. He made no statement but answered questions in a strong voice. His lawyer waived a detention hearing.

Having survived a shot to the head at point-blank range, Giffords, a 40-year-old Democrat, was in critical condition at a Tucson hospital.

Doctors said it was a good sign there was no increased swelling and that she continued to respond to simple commands such as squeezing a finger and wiggling her toes.

"Things are going very well," said Dr Peter Rhee, the hospital's trauma director.

The shootings fueled debate about whether heated political rhetoric like that at the November congressional elections, could fuel violence and if all sides should tone it down.

"Thoughts and prayers"

At the White House, President Barack Obama mourned victims and steered clear of the debate.

"Right now the main thing we're doing is to offer our thoughts and prayers to those who've been impacted, making sure that we're joining together and pulling together as a country," Obama said.

Bowing their heads, Obama and first lady Michelle Obama paused for a moment of silence. A bell tolled three times as an estimated 300 White House staffers offered their respects.

At the US Capitol, security was tightened and flags flew at half staff. Hundreds of congressional staffers observed a moment of silence in honor of the shooting victims.

While the motive for the attack was not yet clear, several facts emerged about Loughner. People who knew him said he was a troubled young man who had been asked to leave a local college for disruptive behavior.

Investigators said they had found an envelope at Loughner's residence with the handwritten phrases "I planned ahead" and "My assassination," along with the name "Giffords" and what appeared to be Loughner's signature.