New York (CNN) -- Osama bin Laden's son-in-law was found guilty Wednesday in New York of helping al Qaeda terrorists conspire to kill Americans and providing material support to terrorists.
Federal prosecutors said at Sulaiman Abu Ghaith's trial that the Kuwaiti cleric played a crucial role as the organization's principal mouthpiece and recruiter, helping "restore al Qaeda's trove of new terrorists" as deadly missions turned its members into martyrs.
Abu Ghaith, wearing a dark suit, displayed no reaction as the verdict from an anonymous jury of nine women and three men was read. The jury deliberated about 5½ hours. As the defendant was being escorted out the packed courtroom, there was a slight smile on his face.
"A jury unanimously found that Sulaiman Abu Ghayth not only conspired to provide, and actually provided, material support to al Qaeda, but also conspired to kill Americans," Manhattan U.S. Attorney Preet Bharara said in a prepared statement. "He was more than just Usama bin Laden's propaganda minister. Within hours after the devastating 9/11 attacks, Abu Ghayth was using his position in al Qaeda's homicidal hierarchy to persuade others to pledge themselves to al Qaeda in the cause of murdering more Americans."
Abu Ghaith's attorney, Stan Cohen, described his client as stoic and at peace.
"He has confidence this is not the end, but the beginning," said Cohen, adding that the defense was confident about an appeal.
Abu Ghaith, 48, faces life in prison at sentencing on September 8.
The government did not allege that Abu Ghaith served an operational function or had a direct hand in carrying out the attacks.
Bharara added: "Like the others who have faced terrorism charges in Manhattan's federal courthouse before him, Abu Ghayth received a fair trial, after which a unanimous jury rendered its verdict, justly holding him accountable for his crimes. We hope this verdict brings some small measure of comfort to the families of the victims of al Qaeda's murderous designs."
In a statement, George Venizelos, assistant director in charge of the FBI's New York field office, likened Abu Ghaith to "a consigliere for the mob or the chief of staff to a corrupt foreign leader" and said he was a "spokesman, confidant and senior adviser" to bin Laden's organization.
Abu Ghaith "looked to better al Qaeda's reputation at every turn, offering advice and counsel to the organization's senior leadership. He encouraged others to abandon the true tenets of their faith, swearing bayat to the twisted ideology of al Qaeda," Venizelos said, using the Islamic word for allegiance.
The trial was held blocks from the site of Manhattan's World Trade Center, and several survivors of the attacks and family members of victims attended.
"Without people like him, al Qaeda dies with every suicide attack," Assistant U.S. Attorney John Cronan said in his closing argument Monday.
Abu Ghaith testified last week that he never joined the ranks of al Qaeda and assisted bin Laden only as a speaker on spiritual topics.
Known in the Persian Gulf region as a religious scholar and fiery orator, Abu Ghaith met the al Qaeda leader in Afghanistan in June 2001, according to testimony. He denied advance knowledge of the 9/11 attacks but said he'd heard talk that "something big" was about to happen. He moved his family from Afghanistan to Kuwait days before the September 11, 2001, attacks on New York's World Trade Center and the Pentagon.
Just after the attacks, the al Qaeda leader summoned Abu Ghaith to his remote mountain hiding place and asked him to help "deliver a message to the world" on the organization's behalf, the religious scholar testified.
After his arrest last year in Jordan, activists and legislators sparred over where to try Abu Ghaith, with some calling for the case to be heard in a military courtroom at Guantanamo Bay, Cuba. Instead, the trial took place amid extra security measures in U.S. District Court in Manhattan.
The jury watched several videos produced after the meeting with the al Qaeda leader, of Abu Ghaith making passionate calls for further attacks on Americans, flanked by bin Laden on one side and an AK-47 rifle on the other.
He called for fellow Muslims to "terrorize the infidels" and vowed the "storm of airplanes will not abate," a phrase he said on the stand was given to him in "bullet points" provided by bin Laden for the speeches.
Cohen, the defense attorney, accused prosecutors of using 9/11 imagery in lieu of evidence of his client's guilt and told the jurors Monday he counted 172 references to bin Laden and 9/11 in the first half of the prosecution's closing argument.
"It was intended to sweep you away in anguish and pain and to ask for retaliation. It was intended to make you look away from the evidence," Cohen told the panel. He said none of the government's witnesses conclusively tied his client to terrorism-related activities, including a terrorism specialist with the FBI and two cooperating former al Qaeda members.
CNN's Ray Sanchez and Lawrence Crook III contributed to this report.