A man described as a spokesman for Osama bin Laden has been arrested and will be tried in New York City, the US has confirmed.
Sulaiman Abu Ghaith was captured within the last week in Jordan, Congressman Peter King said on Thursday.
Mr Abu Ghaith is Bin Laden's son-in-law and played a role in plotting the attacks of 9/11, US officials said.
Bin Laden was killed in a May 2011 raid on his hideout in Pakistan by a team of US commandos.
Mr Abu Ghaith is scheduled to appear in a federal court on Friday on charges of conspiracy to kill United States nationals.
"Sulaiman Abu Ghaith held a key position in al-Qaeda, comparable to the consigliere in a mob family or propaganda minister in a totalitarian regime," said FBI Assistant Director-in-Charge George Venizelos in a statement.
"He used his position to threaten the United States and incite its enemies."
'One by one'
A teacher and mosque preacher in Kuwait, Mr Abu Ghaith was stripped of his Kuwaiti citizenship after 9/11.
Justice department officials say Mr Abu Ghaith served alongside Bin Laden from May 2001 to 2002, speaking on behalf of al-Qaeda and warning that attacks similar to 9/11 would continue.
Specifically, on the morning of 12 September 2001, Abu Ghaith appeared with Bin Laden and Ayman al-Zawahiri to warn the US that a "great army is gathering against you" and called upon "the nation of Islam" to do battle against "the Jews, the Christians and the Americans," according to court records.
He was reportedly smuggled to Iran sometime in 2002.
A Jordanian security official told the Associated Press that Mr Abu Ghaith was handed over last week to US officials under both countries' extradition treaties.
Mr King called the arrest a "very significant victory" in the fight against al-Qaeda.
"One by one, we are getting the top echelons of al-Qaida," the Republican congressman said. "I give the administration credit for this: it's steady and it's unrelenting and it's very successful."
Mr Abu Ghaith's trial will mark one of the first prosecutions of senior al-Qaeda leaders on US soil.
Since 9/11, 67 foreign terror suspects have been convicted in US federal courts, according to data obtained by the group Human Rights First.
Some US lawmakers disagreed with the decision to try Mr Abu Ghaith in New York.
"When we find somebody like this, this close to Bin Laden and the senior al-Qaeda leadership, the last thing in the world we want to do, in my opinion, is put them in a civilian court," said Republican Senator Lindsey Graham on Thursday.
"This man should be in Guantanamo Bay," he said.