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'
US authorities have released little information about Mr Abu Ghaith's capture.
But Turkish newspaper Hurriyet reported that he was caught on his way to Kuwait, shortly after leaving Turkey.
According to the paper, Mr Abu Ghaith had already been taken into custody more than a month ago in the Turkish capital Ankara, but police decided to release him because he had not committed any crime.
Reports also indicate that the FBI took the lead role in the operation, as part of a multi-agency body, the High-Value Detainee Interrogation Group.
The group was set up after President Barack Obama shut down a controversial CIA programme, which had detained suspects in a network of secret prisons under George W Bush's administration.
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 12 September 2001, he 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 with two other senior al-Qaeda leaders in 2002. Unconfirmed reports said they were held under house arrest by the Iran's Islamic Revolutionary Guard Corps (IRGC).
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-Qaeda," 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.
A senior administration official told the BBC that Mr Obama's national security team "unanimously agreed" that prosecution of Mr Abu Ghaith in federal court was in the US' national security interests.
"The administration is seeking to close Guantanamo, not add to its population," the official said.