A federal jury deliberated six hours over two days before finding 48-year-old Sulaiman Abu Ghaith guilty of charges that included conspiracy to kill Americans and providing support to al-Qaida.
Abu Ghaith, a Kuwaiti who married bin Laden's eldest daughter about five years ago, is the highest-ranking al-Qaida figure brought to trial on U.S. soil since the Sept. 11, 2001, attacks.
Prosecutors said he played a powerful role in the terror organization's post-9/11 propaganda videos, in which he and others gloated over the destruction and he warned of a "storm of airplanes" to follow.
He could get life in prison at sentencing Sept. 8.
In a statement, U.S. Attorney Preet Bharara said he hopes the verdict brings some comfort to al-Qaida victims.
"He was more than just Osama bin Laden's propaganda minister," Bharara said. "Within hours after the devastating 9/11 attacks, Abu Ghaith was using his position in al-Qaida's homicidal hierarchy to persuade others to pledge themselves to al-Qaida in the cause of murdering more Americans."
[caption id="attachment_18469" align="alignleft" width="300"] In this courtroom sketch Osama bin Laden's son-in-law, Sulaiman Abu Ghaith, right, testifies at his trial Wednesday, March 19, 2014, in New York, on charges he conspired to kill Americans and aid al-Qaida as a spokesman for the terrorist group. Listening to testimony are Judge Lewis Kaplan, upper left, and clerk Andrew Mohan, center left, as an image of Khalid Sheik Mohammed, the self-professed architect of the Sept. 11 attacks, appears on a video monitor. In his surprise testimony, Abu Ghaith recounted the night of the Sept. 11, 2001, attacks, when bin Laden sent a messenger to drive him into a mountainous area for a meeting inside a cave in Afghanistan. (AP Photo/Elizabeth Williams)[/caption]
Abu Ghaith's lawyers had argued that he was being prosecuted for his words and associations — not his deeds — and that there was no evidence tying him to any of the terror plots that prosecutors suggested he knew about ahead of time.
Attorney General Eric Holder said the verdict was a success for the Obama administration's policy of using the federal courts instead of military tribunals to handle terrorism cases.
"It would be a good thing for the country if this case has the result of putting that political debate to rest," he said.
As the verdict was read, Abu Ghaith appeared composed. He smiled at a friend from Kuwait in the courtroom as he was led away.
Abu Ghaith's attorney, Stanley Cohen, vowed to appeal, complaining that the judge had pressured the jury for a verdict and had barred the defense from calling self-described 9/11 mastermind Khalid Sheik Mohammed as a witness. In a written statement, Mohammed had said Abu Ghaith had no military role in al-Qaida.
In the trial's most dramatic testimony, Abu Ghaith described being summoned to a dark Afghanistan cave within hours of the Sept. 11 attacks to confer with bin Laden, who told him: "We are the ones who did it."
Abu Gaith testified that a worried bin Laden asked him how America would respond.
"America, if it was proven that you were the one who did this, will not settle until it accomplishes two things: to kill you and topple the state of the Taliban," Abu Ghaith said he replied.
Prosecutors alleged that during the meeting, Abu Ghaith agreed to speak on the widely circulated videos used to recruit new followers willing to go on suicide missions like those carried out by the 19 men who hijacked four planes for the 9/11 attacks.
"The storm of airplanes will not stop," Abu Ghaith warned in an October 2001 video played for the jury.
Also shown repeatedly to the jury were frames of a video made the day after 9/11 that showed Abu Ghaith seated next to bin Laden and two other top al-Qaida leaders as they tried to justify the attacks.
On the witness stand, the defendant calmly denied he was an al-Qaida recruiter and claimed his role was a religious one aimed at encouraging all Muslims to rise up against their oppressors.
Prosecutors did not accuse him of any role in 9/11 or any direct knowledge of the plot ahead of time. But he testified that he had been told al-Qaida was about to do "something big."
The conviction "means something. It means there are consequences," said Debra Burlingame, whose brother Charles was the American Airlines pilot of the plane that hijackers crashed into the Pentagon.
Two more major terrorism trials are scheduled for later this year in New York.
In one case set for next month, Egyptian preacher Mustafa Kamel Mustafa faces charges he conspired in 1999 to set up a terrorist training camp in Bly, Ore., and helped abduct two American tourists and 14 others in Yemen in 1998.
In the other, scheduled for November, two defendants extradited from Britain and a third snatched off the streets of Tripoli, Libya, in October will face charges in the 1998 bombings at U.S. embassies in Africa that killed 224 people, including a dozen Americans.
Associated Press writer Jennifer Peltz in New York contributed to this report.