A New York jury on Tuesday found Osama bin Laden's son-in-law guilty of conspiracy to kill Americans and supporting terrorists over his role as Al-Qaeda spokesman in 2001-2002.
Suleiman Abu Ghaith whose three-week trial has been the most high-profile Al-Qaeda case to reach a US federal court, now faces life behind bars.
The 48-year-old preacher from Kuwait stood impassively as the court clerk read out the verdict, declaring him guilty of conspiracy to kill Americans, conspiracy to provide material support and providing material support to terrorists.
Judge Lewis Kaplan said Abu Ghaith, who was brought to the United States only last year after being detained in Jordan, would be sentenced on September 8.
Campaigners seized on the trial as proof that terror cases result in swift justice in civilian courts as pressure builds to close the Guantanamo Bay prison camp.
"There is to be no demonstration by anyone in the court room of any kind," Kaplan told the packed room after the 12-person jury took just four hours of deliberations to return the unanimous verdict.
Afterward, Abu Ghaith touched his heart in thanks, smiling at his lawyers.
Wearing his habitual dark suit, with open-necked white shirt he appeared relaxed despite having pleaded innocent.
- Defense to appeal -
US attorney Preet Bharara welcomed the verdict against a man he said had exploited his position in Al-Qaeda's "homicidal hierarchy" to recruit a new generation of terrorists after 9/11.
"We hope this verdict brings some small measure of comfort to the families of the victims of Al-Qaeda's murderous designs," Baharara said in a statement.
Defense lawyer Stanley Cohen told reporters that he would appeal the verdict, accusing the judge of being "coercive," and taking issue with instructions to the jury and Kaplan's decision not to allow presumed 9/11 plotter Khalid Sheikh Mohammed to testify in court.
He said his client was "stoic" and "at ease."
"He has confidence that this is not the end but the beginning. We think there are a number of compelling issues for an appeal," Cohen said.
US prosecutors alleged that Abu Ghaith was bin Laden's right-hand man as the group's main messenger after the 9/11 attacks.
- Day-after video -
Abu Ghaith is most famous for sitting next to bin Laden in a video on September 12, 2001 as he claimed the attacks on US targets that killed nearly 3,000 people the day before.
In follow-up videos, he threatened a "storm of airplanes," proof the government said that he was implicated in the December 2001 plot to blow up a transatlantic flight from Paris with a shoe bomb.
Video clips, his confession to an FBI agent en route to the United States last year and his testimony on the stand all proved his guilt, prosecutors said.
Assistant US attorney John Cronan said the first thing bin Laden did as the Twin Towers lay smouldering was to ask Abu Ghaith to take Al-Qaeda propaganda global.
"In the most important period of time in Al-Qaeda's savage history Suleiman Abu Ghaith was Osama bin Laden's principle messenger," he said.
"Al-Qaeda is about murdering Americans and regardless of role, participating in that conspiracy in any way whatsoever is a crime. Did that man knowingly participate in that conspiracy? Of course he did."
Cohen argued there was no evidence linking Abu Ghaith to any specific conspiracy, presenting him as a pious imam who "had no problem" with the Americans who liberated his country after the 1990 Iraqi invasion.
Kaplan told the jury that should Abu Ghaith be deemed a conspirator, under US law he would be responsible for any acts that happened before he joined.
Abu Ghaith fled Afghanistan for Iran in 2002. He was arrested in Turkey in 2013 and sent to Jordan, where he was transferred to US custody.