TUCSON, Ariz.—Speaking in a slow, sometimes slurred voice, Jared Loughner, the accused gunman in last year's mass shooting outside a Tucson grocery store, said Tuesday he is guilty of killing six people and wounding 13 others, including the attempted assassination of former Rep. Gabrielle Giffords.

n a Tucson courtroom filled with victims and victims' family members, Mr. Loughner changed his plea to guilty from not guilty on 19 murder and attempted murder counts relating to the January 2011 shooting.

Judge Larry A. Burns read each count, which included the name of a dead or wounded person, aloud to Mr. Loughner, who appeared attentive but emotionless.

"I plead guilty," Mr. Loughner responded to each count, as he looked down at a list of the charges.The guilty plea was expected as part of a deal worked out in advance between prosecutors and defense lawyers. The deal will likely lock Mr. Loughner, 23 years old, in federal prison for life, but save him from a possible death sentence if he had been convicted at trial.

The plea also marked the beginning of closure for Tucson, where a spasm of violence shocked the country 18 months ago and still reverberates in this community. Earlier in the day, Ms. Giffords's husband, astronaut Mark Kelly, said in a statement that he and his wife support the deal.

Former Rep. Gabrielle Giffords, who was shot in the head during a mass shooting in Arizona last year, on Tuesday said she supports a possible plea deal for the accused gunman that would keep him in prison for life. Tamara Audi has details on Lunch Break. Photo: Getty Images.

"The pain and loss caused by the events of January 8, 2011 are incalculable," Mr. Kelly said. "Avoiding a trial will allow us—and we hope the whole Southern Arizona community—to continue with our recovery and move forward with our lives."