Zacarias Moussaoui, the only person charged in the Sept. 11 attacks, was indicted for a third time Tuesday so prosecutors could specify the alleged conduct that would warrant his execution.

The indictment said Moussaoui acted in "an especially heinous, cruel and depraved manner," with premeditation, to cause death and commit terrorism.

The charges were an attempt to undermine a challenge by Moussaoui's court-appointed lawyers to the federal death penalty law. The defense lawyers filed their pleading last week in anticipation of the new indictment.

Prosecutors have told a judge they would seek to execute Moussaoui if he were convicted of conspiracy to commit terrorism and other charges related to the attacks on the World Trade Center and the Pentagon.

They returned to the grand jury because the Supreme Court ruled last month that juries, not judges, must make the crucial decisions on life or death.

The prosecutors acted even though the justices focused on trial juries, not grand juries. However, legal experts said they expect challenges to indictments in capital cases that failed to specify conduct that would trigger the death penalty.

"They probably didn't have to do it but it was very prudent for them to do it," said Michael Mello, professor of law at Vermont Law School and a specialist on capital punishment.

"It's a pre-emptive strike. When the whole word is watching, you want to be extra, extra careful."

Stephen Bright, director of the anti-death penalty Southern Center for Human Rights in Atlanta, said: "What prosecutors have done is cover their base in case the court rules that a grand jury must charge on aggravating circumstances. It's not very hard to get a grand jury indictment."

Moussaoui will be arraigned for a third time on Thursday. In his first arraignment in December, U.S. District Judge Leonie Brinkema entered an innocent plea after Moussaoui said he would make no plea. She entered another innocent plea at his second arraignment in June, explaining that Moussaoui's attempted plea of "no contest" was the equivalent of a conviction.

The indictment said that Moussaoui:

--Participated in an act contemplating a person's life would be taken or intending that lethal force would be used.

--Intentionally engaged in an act of violence knowing that act created a grave risk of death.

--Knowingly created a grave risk of death to one or more persons in addition to victims of the offense.

--Committed offenses in an especially heinous, cruel and depraved manner involving torture and serious physical abuse.

--Committed offenses after substantial planning and premeditation to cause the death of a person and commit an act of terrorism.

Moussaoui is the only individual charged in connection with the attacks. The original indictment in December accused him of plotting with the 19 hijackers and mimicking their conduct, including enrollment in flight schools. While government officials believe he was planning to be the 20th hijacker, Moussaoui was in custody on Sept. 11 on immigration violations.

The second indictment of Moussaoui last month dropped references to allegations that he sought information on crop dusting aircraft and had information on the subject in his computer.

Last week, Moussaoui attorney Frank Dunham Jr. contended the high court ruling should bar the government from seeking the death penalty.

Dunham, told by the court to remain in the case while Moussaoui represents himself, said even a revised indictment should not allow the government to seek Moussaoui's execution. He said only Congress could fix flaws in the federal law as a result of the Supreme Court ruling.

The federal death penalty law, like those in most states, draws a line between crimes that can trigger a death sentence and those that cannot. Murder by itself does not make a defendant eligible for that sentence; there must be what prosecutors call aggravating factors, or circumstances of the crime, which make it particularly egregious.

For example, under the federal system, the killing of a law enforcement officer is a capital crime, as is a killing that results in the destruction of federal property or of private property that is involved in foreign or interstate commerce.

Moussaoui is charged with conspiracy to commit terrorism, pirate aircraft, destroy aircraft, use weapons of mass destruction, murder U.S. government employees and destroy property.