Abu Gaith is married to Fatima, one of Al Qaeda founder Osama bin-Laden's daughters. He is 47 years old.
He was a Muslim preacher and teacher in Kuwait, who spoke against Saddam Hussein's invasion of his country in the early 1990s. He is believed to have met bin Laden in 2000, when he visited Afghanistan.
Abu Ghaith was stripped of his Kuwaiti nationality after appearing in videos defending the September 11, 2001 attacks on the United States and threatening further violence.
A US indictment says he appeared on video with bin Laden and his second in command Ayman al-Zawahri, and warned the U.S. and its allies 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 the indictment released on Thursday, he is to be charged in the US with conspiracy to kill Americans.
But Justice Department officials have described him as a propagandist, who they believe did not have an operational role in al-Qaeda for years and did not participate in the attacks on Sept. 11, 2001.
In 2002, Abu Ghaith went to Iran, one of a handful of al-Qaeda operatives who traveled there after 9/11 attacks.
From there he slipped into Turkey, where he lived for almost a decade. He was detained in Turkey after he allegedly entered the country illegally from Iran. But an Ankara court freed him because he did not commit any crime on Turkish soil.
According to local media, Turkey had hesitated to extradite him to the United States fearing it could become a target of Al-Qaeda.
Earlier this month, Turkey deported him to Kuwait. On a stopover in Jordan, CIA agents seized him and took him to the US.