WASHINGTON — Jared Lee Loughner is set to plead guilty Tuesday in the shooting attack that severely wounded then Rep. Gabrielle Giffords, according to knowledgeable sources, as mental health officials believe he is now competent to understand the charges against him in the assault, which killed six people and injured 13 at a gathering with the congresswoman’s constituents in Tucson.

At the hearing Tuesday morning in U.S. District Court in Tucson, psychiatric experts who have examined Loughner, 23, are scheduled to testify that they have concluded that despite wide swings in his mental capacity, at this time he comprehends what happened and acknowledges the gravity of the charges, according to two sources who spoke on condition of anonymity because the case was still unfolding.

The terms of the plea arrangement remained unclear Saturday on whether Loughner would admit guilt to all or some of the charges in return for a lengthy prison sentence rather than risk a potential death penalty verdict at trial.

State prosecutors in Tucson initially said they would pursue charges against Loughner as well, but the federal government went first. They now would probably review their options and decide whether it would be wise to go forward.

Many of the victims of the Jan. 8, 2011, attack and their families are likely to attend the hearing in downtown Tucson, not far from the site of the attack during Giffords’ Congress On Your Corner event. Survivors would be invited to testify about the assault and their injuries at a separate hearing yet to be scheduled, in which Loughner would be formally sentenced.

Loughner’s agreement to plead guilty, if finalized in court Tuesday, would probably end more than a year and a half of psychiatric evaluations and testing, including some periods in which he was medicated at a federal prison hospital. It also would close out complex legal disagreements between prosecutors and defense attorneys over his mental capacity.

The shooting on a quiet Saturday morning stunned the nation. A man with repeated episodes of bizarre behavior was able to easily acquire a Glock 9-millimeter semiautomatic pistol and ammunition, authorities said, and then open fire at the congresswoman’s event.

Capital punishment initially was on the table when federal prosecutors in Tucson obtained a grand jury indictment against Loughner, and they announced the case as one with “potential death penalty charges.”

The indictment said a search of Loughner’s home turned up a letter hidden in a safe in which Giffords thanked him for attending an earlier Congress On Your Corner event. Also allegedly in the safe was an envelope with handwriting that said “I planned ahead” and “My assassination” and Giffords’ name, “along with what appears to be Loughner’s signature.”

Prosecutors pointed to that as evidence Loughner coldly calculated the attack.

But soon after his arrest, the focus immediately became on whether he was mentally fit to stand trial.

His friends said he dabbled in marijuana and alcohol; while attending a local community college, he videotaped a rant in September 2010 on the campus in which he screamed that the school was not following the Constitution.

According to the indictment, Loughner purchased the Glock at a Tucson gun shop in November 2010, and the ammunition at a Wal-Mart store on the morning of the shooting. He took a taxi to the Safeway parking lot where Giffords was holding her constituent meeting.

Among those killed were 9-year-old Christina-Taylor Green, born on Sept. 11, 2001, and U.S. District Court Judge John M. Roll, the presiding federal judge there. Giffords, though seriously wounded in the head, has slowly been recovering. The Democrat has since resigned from Congress.

Loughner was quickly subdued, and more details came out about his troubled psyche. In July 2011, a prison doctor reported that Loughner was depressed and insisting that “the radio was talking to him and inserting thoughts into his mind.”

He was placed under suicide watch and often paced in circles about his cell, according to court records and pre-trial testimony. He screamed and cried. He remained convinced that Giffords was dead, and became angry when told she survived. Once, advised he might face the death penalty, Loughner sobbed for nearly an hour. “I want to die,” he said. “Give me the injection now. Kill me now.”

According to court records, two medical experts agreed Loughner suffered from schizophrenia and predicted any improvements would be far away, if possible at all. Even when they tried to interview him, they said, he lay in bed and hid under his covers. What statements he did make were nonsensical, often obsessing about treason.

In a May 2011 hearing, federal marshals forcibly removed Loughner from the courtroom after he began shouting and screaming incoherently, something about “kill free” or “kill shot” — words that were hard to understand. Then he yelled, “She died in front of me!”

Given the option of behaving or watching the proceedings from a nearby cell with a remote television screen, Loughner chose the cell.