At least five people, including a child, were killed and 12 others were injured when a man opened fire indiscriminately at a constituency meeting held by Congresswoman Gabrielle Giffords in Tuscon, Arizona.
Ms Giffords, a 40-year-old Democrat in her third term in the House of Representatives, was airlifted to a hospital in Tucson after being shot at point-blank range at a Safeway supermarket.
The doctor treating her said the congresswoman was in a critical condition but that he was optimistic about her recovery.
Dr Peter Rhee told a news conference at Tuscon's University Medical Center that one child had died and nine other victims were being treated there for shooting wounds.
President Barack Obama confirmed that five people had been killed in the attack, including federal judge John Roll and a 9-year-old girl and said Ms Giffords was battling for her life.
A federal law enforcement official identified the suspected gunman as 22-year-old Jared Loughner of Tucson.
The official said the suspect was had been tackled after the shooting and was in custody.
Ms Giffords was hosting a ‘Congress on Your Corner' event - public gatherings to give her constituents a chance to talk directly with her – when she was attacked from about four feet away, local media said.
The suspect then tried to run away but was tackled by a bystander.
‘We do not yet have all the answers. What we do know is that such a senseless and terrible act of violence has no place in a free society. I ask all Americans to join me and Michelle in keeping Representative Giffords, the victims of this tragedy, and their families in our prayers,’ Mr Obama said.
Ms Giffords, who was re-elected in November, has focused on immigration reform, military issues, stem cell research and alternative energy while serving in Congress.
She is married to US astronaut Mark Kelly.