Osama bin Laden's son-in-law, the voice of fiery al-Qaida propaganda videotapes after the September 11 attacks, was convicted Wednesday of conspiring to kill Americans for his role as the terror group's spokesman.

The verdict came after about five hours of deliberation in the case against Suleiman Abu Ghaith, the highest-ranking al-Qaida figure to face trial on US soil since the attacks. The Kuwaiti imam had testified during a three-week New York trial that he answered bin Laden's request in the hours after the attacks to speak on the widely circulated videos used to recruit new followers willing to go on suicide missions like the 19 who hijacked four planes on September 11, 2001.

"The storm of airplanes will not stop," Abu Ghaith was heard warning in an October 2001 video that was played for the jury.

Also shown repeatedly to the jury during the trial were frames of a video made September 12, 2001, that showed Abu Ghaith seated next to bin Laden and two other top al-Qaida leaders as they tried to justify the attacks.

On Monday, during closing arguments, assistant US attorney John Cronan underscored the importance of Abu Ghaith's post 9/11 status.

"Going to that man was the very first thing Osama bin Laden did on 9/11 after the terror attacks," he said. "The defendant committed himself to al-Qaida's conspiracy to kill Americans, and he worked to drive other people to that conspiracy."

He added: "During the most important period of time in al-Qaida's savage history, Suleiman Abu Ghaith was Osama bin Laden's principal messenger … He used his fiery oratory to incite al-Qaida's growing army of terror in this war with America."

The jury returned a verdict on three charges: conspiracy to kill Americans, conspiring to provide support to al-Qaida and providing support to al-Qaida. The charges carry a potential penalty of life in prison.

Captured in Jordan last year and brought to New York, Abu Ghaith has actively participated in his trial. He listened to testimony and arguments through headphones linked to an Arabic translator.

Taking the witness stand in his own defense, Abu Ghaith in a calm voice 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. He insisted he agreed to meet with bin Laden in a cave on the night of September 11 out of respect for bin Laden's standing as a sheikh.

"Despite knowing that he was responsible for the deaths of hundreds of Americans, you met with him to be polite, correct?" prosecutor Michael Ferrara asked on cross-examination.

"I didn't go to meet with him to bless if he had killed hundreds of Americans or not. I went to meet with him to know what he wanted," Abu Ghaith said.

Government witnesses included an FBI agent who interviewed him on a 10-hour flight to the United States and a Buffalo-area man who testified that he heard him speak in Afghanistan several months before the September 11 attacks.

Another witness was a British man who was supposed to join Richard Reid in a shoe-bomb attack in December 2001 but who quit the conspiracy during a trip home after his parents warned him that he better not be a terrorist. He did not know Abu Ghaith, but prosecutors introduced the evidence to show Abu Ghaith knew what he was talking about when he warned that air attacks against the United States would continue.

Abu Ghaith's defense lawyer, Stanley Cohen, argued there was "zero evidence" that the 48-year-old former teacher knew of the conspiracies the government claimed he knew about. Citing the videos of his client, Cohen warned jurors not to let prosecutors "intimidate you and to frighten you into returning verdicts not based upon evidence, but fear."

Those videos, though, emerged as the centerpiece of the government's case. One – titled 'Convoy of Martyrs' – features Abu Ghaith preaching over horrific footage of a plane flying into one of the World Trade Center towers. Another showed the defendant looking at bin Laden admiringly as the al-Qaida leader boasted that he knew the attack would make both towers fall.

Prosecutors used Abu Ghaith's testimony and closing arguments as an opportunity to reshow the image of the burning towers to jurors.

In his closing, Cronan used the defendant's testimony against him.

"Accept it as totally true. If you do that, he's guilty," Cronan said.