A federal jury convicted a Falls Church man yesterday of plotting to kill President Bush, concluding that Ahmed Omar Abu Ali joined an al Qaeda conspiracy to mount a series of Sept. 11-style attacks and assassinations in the United States.

The trial in U.S. District Court in Alexandria was the first in an American criminal courtroom to rely so heavily on evidence gathered by a foreign intelligence service. Security officers from Saudi Arabia, where Abu Ali was jailed for 20 months, provided the bulk of the government's case, testifying via video from the kingdom.

The successful prosecution could smooth cooperation in terrorism investigations between U.S. and other intelligence services, which normally are reluctant to allow their officers to testify in a U.S. criminal case.

"It's a highly significant case in terms of a new kind of cooperation," said Stephen Saltzburg, a law professor at George Washington University. He added that the conviction could also make the government more likely to try terrorism cases in civilian courts rather than military tribunals.

Jurors convicted Abu Ali, 24, a U.S. citizen, on all nine counts against him, including conspiracy to assassinate the president, conspiracy to commit aircraft piracy and providing material support to al Qaeda. He faces 20 years to life in prison when he is sentenced Feb. 17.

The verdict, on the third day of deliberations, brought a quiet end to one of the most emotional terrorism cases since the Sept 11, 2001, attacks. Abu Ali's parents had mounted a highly public campaign to have him brought back from Saudi Arabia. They alleged that their son was tortured by Saudi security officers and that U.S. officials were complicitous in the treatment.

That claim became the center of Abu Ali's defense: that his admission to Saudi security officers about being involved in the terror plot was coerced. But first a judge and then a jury rejected that argument.

Juror Nancy Ramsden said the panel did not believe Abu Ali's allegation. The key piece of evidence, she said, was his 13-minute videotaped statement. "He was laughing; he was joking. It was chilling," she said. "He was leaning back, rocking in his chair, asking for water, laughing, smiling. He wasn't moving as though he was in pain. . . . It didn't appear he was being coerced."

The Saudi security officers, including the warden of the prison where Abu Ali said he was tortured, testified in Arabic that he was treated well and had made his statement willingly. The warden, whose name was not disclosed for security reasons, said prisoners receive three meals a day and are even allowed to order from restaurants because the prison has no central kitchen.

Ramsden said last night that the jury found the Saudi officers believable though "painful to watch" at times because of language and cultural barriers. "It would have been more comforting to have more U.S.-backed information," she said. "But I don't know how that could have happened. The fact is, this all happened in Saudi Arabia."

Abu Ali's family members, who attended nearly all of the 21/2-week trial, filed quietly out of the courtroom yesterday and declined to comment. Abu Ali showed no reaction when the verdict was read and smiled as he was led out of court.

Shaker Elsayed, the imam at Dar Al Hijrah mosque in Falls Church, where Abu Ali once worshiped, said the verdict "will not stand. There will be an appeal."

Defense attorneys also indicated that they plan to appeal. "Obviously, the jury has spoken, but the fight is not over," said Khurrum Wahid, an attorney for Abu Ali. "We plan to continue to use the justice system to pursue our client's innocence."

Prosecutors said the conviction represented a major victory in the campaign against terrorism. "The evidence presented in this case firmly established Abu Ali as a dangerous terrorist who posed a grave threat to our national security," U.S. Attorney Paul J. McNulty said in a statement.

Abu Ali was arrested by Saudi security officers in June 2003 on suspicion that he was connected to the May 12, 2003, bombing of three residential compounds in Riyadh. The bombing killed 23 people.

His incarceration triggered a flurry of legal and diplomatic activity. Abu Ali's parents sued the U.S. government in U.S. District Court in Washington, contending that it had condoned the torture of their son and asking that he be returned to the United States.

The government brought him back and formally charged him with conspiring to kill the president. Prosecutors said he admitted to his Saudi jailers that he joined an al Qaeda cell while studying at a Saudi university and planned to kill Bush by shooting him on the street or blowing him up with a car bomb. Law enforcement sources have said that the plot to kill Bush never advanced beyond the talking stage.

The torture issue took center stage at a hearing last month, when defense attorneys asked a judge to throw out Abu Ali's confession, saying it was obtained under duress. The primary areas of dispute were whether marks on Abu Ali's back were caused by whipping and whether he suffered post-traumatic stress disorder from the alleged abuse.

Abu Ali testified at the pretrial hearing, describing how his Saudi captors chained him to the floor of an interrogation room, shackled his feet and whipped him until his back turned bloody while they screamed, "Confess! Confess!"

But U.S. District Judge Gerald Bruce Lee ruled that he found the story "implausible" because Abu Ali could not describe what he was hit with and because he did not seem to be in pain just a few days later. Lee refused to throw out the confession but said the defense could raise the torture argument again at trial.

Defense attorneys portrayed Abu Ali as a polite young man from Virginia who went to Saudi Arabia only to pursue religious studies. He did not testify at the trial.

Staff writer Mary Beth Sheridan contributed to this report.