Six people were killed and a United States congresswoman seriously injured when a gunman opened fire at a public event in Arizona.
US Representative Gabrielle Giffords was shot in the head at close range while at a constituency meeting outside a store in Tucson.
Pima County Sheriff Clarence Dupnik told a news conference that the suspected gunman has "a troubled past" and used a semi-automatic pistol.
He said that the suspect was tackled to the ground after the shooting that left six people dead and 13 wounded.
Mr Dupnik said the suspect has a troubled past, may have mental issues, and did not act alone.
He said authorities are seeking a second man in connection with the shooting.
Police believe that Ms Giffords was specifically targetted and say a suspicious package has since been found at her office.
President Barack Obama said Ms Giffords was fighting for her life, and he was hopeful she would pull through.
"Among [those who died] were federal judge John Roll, who has served America's legal system for almost 40 years, and a young girl who was barely 9 years old," the president said.
The President has sent FBI Director Robert Mueller to Arizona to oversee the investigation.
Mr Obama said it was not yet known what provoked the attack.
Local media said the arrested man was a 22-year-old Arizona resident.
A surgeon at the University Medical Center in Tucson said Ms Giffords was shot once in the head. Dr Peter Rhee said surgery had been completed and he was optimistic about her condition.
The hospital had received 10 patients, five of whom were in critical condition, he said.
Broadcaster NBC reported a federal judge was among the dead, while CNN said a senior aide to the congresswoman had been killed.
Fox News reported three of the Democratic congresswoman's staff members were shot.
Eyewitness Jason Pekau, who works in a nearby shop, says he heard at least 15 shots, and saw Ms Giffords being taken away on a stretcher.
Healthcare bill support unpopular
Gabrielle Giffords said in March last year that she had faced a deluge of threats over her support of President Obama's landmark healthcare overhaul.
A window in her Tucson office was smashed after Congress passed the legislation.
Ms Giffords, 40, took office in January 2007, emphasising issues such as immigration reform, embryonic stem-cell research, alternative energy sources and a higher minimum wage.
She serves on several congressional committees, including those covering the armed services and foreign affairs. She is married to US astronaut Mark Kelly.
The House of Representatives majority leader Eric Cantor announced that all of next week's legislative debates would be postponed to allow politicians to "take whatever actions may be necessary in light of today's tragedy".