TUCSON — Jared L. Loughner pleaded guilty on Tuesday to killing six people and wounding 13 others last year during a meet-and-greet event here held by Gabrielle Giffords, then a member of the House of Representatives and the primary target of his rampage. The plea brought a sudden resolution to a case that seemed threatened by the fragility of Mr. Loughner’s mental state.

Mr. Loughner, 23, delivered his admission in a slurred monotone — “I plead guilty” — looking straight ahead from his seat at the defendant’s table, his back arched and his hands clasped in his lap. He repeated the words 19 times, one for each of the counts to which he had agreed to plead guilty as part of a deal that will keep him in prison for the rest of his life.

He seemed subdued and resigned, telling Judge Larry A. Burns, who has presided over the case in Federal District Court, that he understood the consequences of his actions, as well as the implications of his plea, which offers him no chance of appeals.

At the hearing, Dr. Christina Pietz, a psychologist who treated Mr. Loughner at a federal hospital in Springfield, Mo., said his feelings had evolved — from regret for failing to kill Ms. Giffords, whom he had harbored a secret grudge against for several years, to remorse for wounding her and others and for taking people’s lives.