Saad bin Laden, a Saudi who is about 22, is suspected by U.S. authorities of helping organize some of Al Qaeda's operations during the last few months, providing the financial and other logistical support necessary to set attacks in motion, one official said on condition of anonymity.
He is suspected of ties to Al Qaeda's first successful operation after the Sept. 11 attacks — the suicide bombing of a synagogue in Tunisia that left 19 dead, most of them German tourists.
Like his father, Saad bin Laden is believed to be in the Afghanistan-Pakistan region. It is unclear if he is with Usama bin Laden, whose whereabouts and condition remain unknown.
Defense Secretary Donald H. Rumsfeld said Monday that even if Usama bin Laden is killed, lieutenants like his son will carry on and the terrorist network will remain a threat.
"There are three, four, five, six, seven people who could — who know where the bank accounts are, who know the key players, the key planners, and are perfectly capable of running that operation," he told reporters during a visit to Suffolk, Va., to view a training exercise. "Whether the son ends up being one of them, one never knows until ... that takes place."
Another official attributed Saad's rising star to simple blood ties: When most of the world is hunting for him, Usama bin Laden can trust his children.
His stature began increasing in the months after the United States went to war on Al Qaeda in Afghanistan on Oct. 7, the counterterrorism official said.
It has accelerated even as the terrorist network decentralized its power structure. This move, under way since earlier this year, gives commanders in the field more authority to conduct operations without guidance from bin Laden's inner circle.
U.S. officials have no evidence that Saad bin Laden played a role in the Sept. 11 attacks, which left 3,000 dead. However, he is believed to be close to Khalid Shaikh Mohammed, whom U.S. officials suspect of masterminding the World Trade Center and Pentagon attacks.
Mohammed — who is said to outrank Saad bin Laden in Al Qaeda's pecking order — has also been linked to the Tunisia bombing, U.S. officials have said.
However, U.S. authorities have no evidence that Saad bin Laden is positioned to take over Al Qaeda in the event of his father's death or capture. Older, more experienced deputies like Ayman al-Zawahri, bin Laden's Egyptian doctor, are more capable of doing so right now, officials said.
But terrorism experts say Usama bin Laden may be looking for a blood relative to carry on the fight, as well.
Usama bin Laden, himself in his mid-40s, has at least 23 children by several wives.
"Some of them share his ideology," the counterterrorism official said. "[Saad] is definitely a believer."
At least two other sons have taken up their father's cause. Mohammed bin Laden, who is around 19, is one of his father's bodyguards, a U.S. official said.
Son Mohammed married the daughter of Al Qaeda's Egyptian military chief, Mohammed Atef, in January 2001 in Kandahar, Afghanistan. Usama bin Laden, Atef, and al-Zawahri all attended. Atef was killed by U.S. airstrikes in Afghanistan last November.
Another son, Ahmed, is believed to be older than Saad and also supports his father's activities.
Usama bin Laden's extended family, one of Saudi Arabia's richest, disowned him in 1994.