President Barack Obama said Rep. Gabrielle Giffords was "battling for her life" and that five others were dead after a shooting attack at a public event in Tucson this morning.
A doctor treating the Arizona congresswoman said he was "very optimistic" about her chances to recover after she had been shot "through and through" the head.
Dr. Peter Rhee, trauma director of the University of Arizona Medical Center, said Giffords, 40, was in critical condition following surgery but was able to "follow commands" in the hospital.
Speaking from the White House, Obama said it was not known what provoked the "unspeakable act" but said, "We are going to get to the bottom of this and we will pull through this."
A total of 18 people were shot, said Pima County Sheriff's Deputy Chief Rich Kastigar.
He identified the suspect as a Jared Lee Loughner, 22, from Arizona and said he was armed with a pistol that had "an extended magazine."
Among the dead is U.S. District Judge John Roll, the chief federal judge in Arizona, and a nine-year-old girl
Obama said he had dispatched FBI Director Robert Mueller to Tucson to help lead the investigation.
The U.S. Capitol Police said it was working with federal, state and local law enforcement agencies and that it has contacted House members "advising them to take reasonable and prudent precautions regarding their personal safety and security."
Giffords was holding an event outside a Safeway grocery store when a man ran up and started shooting, witnnesses said.
