At least five people were killed and Rep. Gabrielle Giffords was shot in the head in the episode, which unfolded as the Democratic congresswoman was greeting constituents in her district in Tucson, Arizona.
Police say the shooter is in custody. He has been identified by officials familiar with the investigation as Jared Loughner, 22.
Officials say Loughner told authorities at the scene he had acted alone, although it is not certain that is the case.
The officials spoke on condition of anonymity, saying they are not authorized to provide details.
Giffords underwent surgery after the attack. The dead include a federal judge and a nine-year-old girl.
