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 had seemed threatened by the fragility of Loughner’s mental state.

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 US 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.

in court

At the hearing, Dr. Christina Pietz, a psychologist who treated Loughner at a federal hospital in Springfield, Mo., said his feelings had evolved — from regret for failing to kill 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.

“I especially cried for the child’’ and ‘‘yelled a lot because it hurt so bad,’’ Loughner once told Pietz, she testified, reading from notes she had kept of their encounters.

His plea brought a measure of victory to prosecutors, who were able to take Loughner off the streets without having to face the uncertain outcome of a trial, where they risked the possibility that his lawyers might sway a jury with an insanity defense.

“We feel that this is a certain and just and appropriate resolution in this case,’’ John Leonardo, the US attorney for Arizona, said outside the courthouse.

Among the survivors, as well as relatives and friends of those whom Loughner killed, there were mixed emotions.

“I truly believe that justice was done today,’’ said Ron Barber, a senior aide to Giffords who was wounded in the shooting and who won a special election in June to fill the remainder of her term after she retired.

To Suzi Hileman, though, whom Loughner shot multiple times, the plea brought her no closer to healing.

‘‘This is too little, too late,’’ said Hileman, who had taken the youngest of the shooting’s victims, 9-year-old Christina-Taylor Green, to Giffords’s event. ‘‘Six people are dead, and my congresswoman had a bullet through her head. This is with me forever.’’

Loughner arrived here on Monday from the hospital in Missouri, where he had been held for more than a year, and spent the night at a medium-security prison before Tuesday’s hearing.

He looked pale and skinny under a khaki jumpsuit, and he offered short answers to the questions Burns asked.

The judge said, ‘‘Has anyone put unfair pressure on you’’ to plead guilty?

“No,’’ Loughner answered.

His mother, Amy Joanne Loughner, wept quietly from a corner of the courtroom.

Loughner began exhibiting odd behavior long before the shooting.

Classmates at Pima Community College described him as strange and eccentric; professors spoke of his ‘‘disorganized thought process,’’ Pietz said.

Once, he asked his parents if they could hear the same voices he had been hearing, she testified.

In written answers to her questions, his parents said they were worried he would kill himself.

In videos he made, Loughner said that he felt depressed and that he had the urge to kill someone.

On Jan. 8, 2011, he fired 31 shots from a 9mm semiautomatic pistol until he was tackled by onlookers as he tried to reload.

It took him 16 seconds to carry out the shooting. He originally faced 49 criminal charges, but most of them were dropped as part of the plea agreement.

Giffords did not attend the hearing. Her husband, Mark E. Kelly, said they had been in contact with the US attorney’s office, informed of every step in the negotiations.

‘‘The pain and loss’’ caused by the shooting ‘‘are incalculable,’’ Kelly said. ‘‘Avoiding trial will allow us, and we hope the whole Southern Arizona community, to continue with our recovery and move forward with our lives.’’

Loughner’s formal sentencing is scheduled for Nov. 15.