TUCSON - The mass shooting Saturday morning that gravely wounded Rep. Gabrielle Giffords (D-Ariz.) and killed a federal judge raised serious concerns that the nation's heated political discourse had taken a dangerous turn.

Police are holding a 22-year-old man in the shooting rampage, which occurred outside a supermarket where Giffords was greeting constituents. The gunman shot Giffords in the head at close range and then continued to fire into the small gathering of people, police said.

Police said they think that Giffords was the target of the attack.

Law enforcement and medical officials in Arizona said that at least 18 people were shot in the melee and that six of them had died, including John M. Roll, the chief U.S. District judge in Arizona, and Gabe Zimmerman, Giffords's local director of community outreach. Also killed was Christina Taylor Green, 9, who was born on Sept. 11, 2001, and had gone to the event with a neighbor. Two other Giffords staffers, district director Ron Barber and community outreach aide Pam Simon, were wounded.

Authorities said they were seeking a second man as a "person of interest" who might have been at the scene with the gunman. He is not a suspect in the shooting, authorities said.

It was unclear what might have motivated the suspect, identified as 22-year-old Jared Loughner. On YouTube, an individual using the same name had posted convoluted videos with a vague anti-government message, that law enforcement officials said they were analyzing. As of late Saturday, Loughner wasn't cooperating with investigators.

In an emotional news conference late Saturday, Pima County Sheriff Clarence W. Dupnik (D) denounced the nation's vitriolic political climate and noted Arizona's part in the rancor after a controversial crackdown on illegal immigration.

"The anger, the hatred, the bigotry that goes on in this country is getting to be outrageous, and unfortunately Arizona has become sort of the capital," Dupnik said. "We have become the mecca for prejudice and bigotry."

The fiery rhetoric that has taken hold in politics, Dupnik said, "may be free speech, but it's not without consequences."

President Obama dispatched FBI Director Robert S. Mueller III to the scene, and U.S. Capitol Police, charged with protecting members of Congress, urged House members to take "reasonable and prudent precautions" regarding their personal safety.

Peter Rhee, trauma director at University Medical Center in Tucson, where Giffords was listed in intensive care, said a single bullet had struck Giffords in the head and traveled through her brain. Sources close to Giffords said the lawmaker was responsive when she was airlifted from the scene and before surgery.

Despite cautious optimism about Giffords's condition, former U.S. surgeon general Richard Carmona, a family friend of Giffords's, told reporters Saturday night that she could need further surgery. "This is a very devastating wound," Carmona said.