A New York court has been told that Suleiman Abu Ghaith was Osama bin Laden's right-hand man.
Mr Abu Ghaith, a son-in-law of Osama bin Laden, proclaimed the 11 September 2001 attacks were al Qaeda's doing and called on others to fight for the Islamist militants, a prosecutor told jurors at the start of his trial today.
A Kuwaiti, the 48-year-old is one of the highest-ranking figures linked to al Qaeda to face a civilian jury on terrorism-related charges since the attacks that destroyed New York's World Trade Center, which stood just blocks from the courthouse where he is on trial.
Assistant US Attorney Nicholas Lewin told the jury during his opening statement that Mr Abu Ghaith "sat at the right hand of Osama bin Laden" after the attacks and bin Laden "asked him to deliver al Qaeda's murderous decree to the whole world."
A lawyer for Mr Abu Ghaith told the Manhattan federal court jury that the US government had "no evidence" his client was aware of plots against Americans.
Mr Abu Ghaith is charged with conspiring to kill Americans, providing material support and resources to terrorists and conspiring to provide material support and resources to terrorists. He faces life in prison if convicted.
Bin Laden, a founder of al Qaeda, was killed in May 2011 by US forces at his hideout in Pakistan.
Bearded and wearing a dark suit with a blue tie, Mr Abu Ghaith occasionally glanced at jurors as he listened to an interpreter translate the proceedings through headphones.
His lawyer, Stanley Cohen, warned jurors that the government would try to overwhelm them with videos and questionable witnesses and said there was no evidence against his client.
Mr Cohen said Mr Abu Ghaith was a "talker" and an ideologue whose recorded statements will horrify jurors and make them "flinch."
But after accumulating "millions of documents" and conducting thousands of interviews over a 13-year investigation, the government cannot prove that Mr Abu Ghaith was aware of any plots against the US, Mr Cohen said.
"At the end of the day, there is really no evidence," Mr Cohen said.
"There is the substitution of evidence with fright."
Specifically, the government contends Mr Abu Ghaith spent time in Afghanistan with bin Laden soon after the attacks and recorded several statements threatening further attacks against Americans, including one that said "the storm of airplanes will not stop."
Four passengers aircraft were hijacked so they could be flown into buildings and nearly 3,000 people were killed.
Mr Lewin said the government would introduce testimony via video feed from a former al Qaeda member in Britain that Mr Abu Ghaith was aware in advance of the group's failed attempt to blow up airplanes with explosives hidden in shoes.
The witness is expected to be Saajid Badat, who plotted with Richard Reid, the man who became known as the shoe bomber after his attempt to detonate explosives on a flight to Miami in 2002.
Reid, a Briton, pleaded guilty to terrorism charges in a US court.
Badat did not follow through with the plan but was sentenced to 13 years in prison for the plot.
While Mr Cohen did not name Badat, he attacked his credibility, saying he was a "mass murderer wannabe" who is under indictment in the US.
"They sit around, the good old boys, and talk, and he tells them what they want," Mr Cohen said.
Prosecutors entered into evidence an October 2001 video showing Mr Abu Ghaith with bin Laden and Ayman al-Zawahiri, the Egyptian doctor who is now considered a leader of al Qaeda.
Mr Cohen asked the prosecution's first witness, FBI agent James Fitzgerald, whether or not he knew an AK-47 rifle near Mr Abu Ghaith in a video was "a prop." Mr Fitzgerald said he did not know.
US District Judge Lewis Kaplan is presiding over the trial, which is expected to finish at the end of March.
In 2010, he oversaw the trial of Ahmed Ghailani, the first detainee from the US military base at Guantanamo Bay, Cuba, to be tried in civilian court.
Ghailani was acquitted of all but one charge over the 1998 US embassy bombings in Africa and was sentenced to life in prison.