Disturbed gunman Jared Lee Loughner has pleaded guilty to carrying out a deadly Arizona mass shooting in January 2011, when he failed to assassinate US Congresswoman Gabrielle Giffords.

Mr Loughner has been receiving psychiatric treatment for schizophrenia for more than a year on court order

Six people were killed in the shooting and 13 were injured, including the congresswoman.

By pleading guilty, Mr Loughner, 23, will avoid the death penalty but still faces a life sentence without the possibility of parole, the prosecution explained during the hearing at a federal court in Tucson.

He waived all right to appeal.

Dressed in a tan prison jumpsuit, his hair cut short, Mr Loughner appeared calm and showed little emotion as he answered the judge’s questions. Asked if it was true he had approached Ms Giffords intending to kill her, Mr Loughner replied: “Yes, it is.”

His lawyer, Judy Clark, said that her client had agreed to plead guilty fully “knowingly, voluntarily and intelligently.” A few minutes earlier, Federal Judge Larry Burns had declared the defendant mentally competent to understand and admit to the charges against him.

Mr Loughner has been receiving psychiatric treatment for schizophrenia more than a year on the court’s orders. He had previously pleaded not guilty, but Judge Burns decided at the time he was not competent to stand trial.

On Tuesday government psychologist Christina Peitz testified that Mr Loughner is psychotic but not deluded, and said the gunman had expressed disappointment that he was a “failure” for not having killed Giffords and others.

Dr Pietz said his condition had improved markedly since he was ordered into treatment in 2011. She added that Mr Loughner ’s mental state would not prevent him from participating in his own defence.

Mr Loughner opened fire on January 8, 2011, outside a Tucson supermarket where Ms Giffords, a Democrat, was meeting with a number of constituents. Among those killed was a nine-year-old.