Osama bin Laden's son-in-law is likely to have vast knowledge of al Qaida's central command, intelligence experts say.
But despite Sulaiman Abu Graith's role as the terror network's leading spokesman, he may not have much useful information about current threats or plots.
On Friday, the charismatic fundraiser pleaded not guilty to conspiring to kill Americans in propaganda videos that warned of further assaults against the US as devastating as the September 11, 2001, attacks on the World Trade Centre and the Pentagon that killed nearly 3,000 people.
Believed to be more of a strategic player in bin Laden's inner circle than an operational plotter, Abu Ghaith would be the highest-ranking al Qaida figure to stand trial on US soil since 9/11.
Intelligence officials say he may be able to shed new light on al Qaida's inner workings - including its dealings in Iran over the past decade. However, experts believe he will probably have few details about specific or imminent ongoing threats. He gave US officials a 22-page statement after his February 28 arrest in Jordan, according to prosecutors. They would not describe the statement.
Bearded and balding, Abu Ghaith said little during the 15-minute hearing in US District Court in New York - in lower Manhattan just blocks from Ground Zero - and displayed none of the finger-wagging or strident orations that marked his propaganda in the days and months after 9/11.
Through an interpreter, he was asked whether he understood his rights, and he nodded yes. Asked whether he had money to hire a lawyer, he shook his head and said no. Bail was not requested, and the judge promised to set a trial date when the case returns to court on April 8.
The fact he is being tried in federal district court is controversial. Republicans are criticising the Obama administration for bringing Abu Ghaith to New York instead of sending him to the military detention centre at Guantanamo Bay, Cuba. President Barack Obama has promised to close Guantanamo, where terror detainees generally have fewer legal rights than in a US federal court.
A month after 9/11, Abu Ghaith called on every Muslim to join the fight against the US, declaring: "Jihad is a duty. The Americans must know that the storm of airplanes will not stop, God willing, and there are thousands of young people who are as keen about death as Americans are about life."
Abu Graith is married to bin Laden's eldest daughter Fatima, and his impassioned rhetoric, which helped al Qaida recruit followers and raise money, made him a natural choice as his spokesman and key adviser, said Tom Lynch, a senior research fellow at National Defence University.