The United States is offering up to $1 million for information that would bring to justice Hamza bin Laden, the son of late al-Qaeda leader Osama bin Laden.
Hamza, 30, is emerging as a leader in the al-Qaeda franchise, the State Department says.
He has released audio and video messages on the internet, calling on his followers to strike the United States and its Western allies, and threatened attacks against the United States in revenge for the May 2011 killing of his father by US military forces.
Hamza married the daughter of Mohammed Atta, the lead hijacker of al-Qaeda's September 2001 airline terrorist attacks on the United States that killed nearly 3,000 people and injured over 6,000.
Osama bin Laden was grooming Hamza to succeed him someday, according to Osama's letters seized from the Abbottabad, Pakistan, compound where he was killed.
Where is Hamza?
The location of Hamza has been the subject of speculation for years with reports of him living in Pakistan, Afghanistan, Syria or under house arrest in Iran.
He is believed to have spent years in Iran alongside his mother, despite al-Qaeda's strident denunciations of the Shia branch of Islam that dominates the country.
Some observers say that the regime in Tehran kept him under house arrest as a way to maintain pressure on rival Saudi Arabia as well as on al-Qaeda, dissuading the Sunni militants from attacking Iran.
However, others say that Iran's Islamic Revolutionary Guard Corps (IRGC) has strong ties with al-Qaeda, as evidenced by Iran's hosting of a large number of al-Qaeda elements on the run from international authorities.
One of Hamza bin Laden's half-brothers told the London-based newspaper The Guardian last year that Hamza's whereabouts were unknown but that he may be in Afghanistan.
Over the past years, al-Qaeda has been struggling to maintain relevance in the public eye.
On the 17th anniversary of 9/11, the organisation resorted to issuing a public statement reiterating its responsibility for the atrocities and slamming all those who believe otherwise.