The dead in the shooting outside a Tucson grocery included U.S. District Judge John Rolf, Gabe Zimmerman, a Giffords aide, and a 9-year-old girl who died at a Tucson hospital from her injuries, CNN reported. The Pima County Sheriff's Office said six people were killed and 13 wounded.
Giffords, D-Ariz.,was shot in the head at close range and was still in critical condition at 6 p.m. Saturday.
Pima County Sheriff Clarence Dupnik, at a news conference in Tucson, identified the suspect as Jared Lee Loughner, 22, of Tucson. Dupnik also said police were searching for a "person of interest" who may have driven Loughner to the scene and aided him in other ways.
The Arizona Republic reported Loughner posted a number of disturbing messages on his MySpace account and on YouTube. A few hours before the shooting outside a Tucson grocery, he posted a farewell on MySpace: "Goodbye. Dear friends . . . Please don't be mad at me."
One posting on YouTube said "Don't trust the current government, listener."
The White House was monitoring the situation, and FBI agents have been dispatched to the scene, CNN reported.
Giffords was holding an event outside a Safeway supermarket when the shooting started.
The congresswoman had previous brushes with violence. Her office was vandalized in March after she voted for the Obama healthcare plan, shots were fired at an Arizona office during the 2008 campaign and an armed protester was removed by police from a 2009 event.
Meanwhile, in Washington, Capitol Police released a statement advising all members of Congress to take "prudent precautions" for their personal security.