Most wanted: America redoubles efforts in its search for new public enemy No 1

(GETTY)

The United States has stepped up the hunt for Ayman al-Zawahiri, the likely successor to Osama bin Laden, with intelligence experts hunting for clues in the trove of digital data seized from the lair where the former al-Qa'ida leader was killed.

In addition to hunting for clues about Mr Zawahiri, an Egyptian-born surgeon known as bin Laden's top lieutenant, they were scouring for information on any pending attacks on the US, with analysts warning that Mr Zawahiri may try to assert his leadership.

The search comes as Barack Obama made a "categorical" ruling against releasing pictures of Bin Laden after he was shot and killed by US Navy Seals inside his lair in Pakistan.

Mr Obama first revealed his decision yesterday during a taping of an interview with CBS News to be aired on Sunday. He said withholding the photographs might encourage conspiracy theorists who claim Bin Laden still lives, but added: "We don't think that a photograph in and of itself is going to make any difference... the fact of the matter is you won't see Osama bin Laden walking on this earth again."

While Mr Obama prepared to build on the political benefits of the operation with a visit to Ground Zero in Manhattan, the White House tried to contain the controversy triggered by the admission that Bin Laden had been unarmed when he was shot.

Testifying on Capitol Hill, Eric Holder, the US Attorney General, warned of short-term retaliation from al-Qa'ida and said he had issued warnings to top law officials across the US to ensure the utmost vigilance. He also pushed back against suggestions the Navy Seals may have erred in executing Bin Laden even though he had no gun and said the killing "was an act of national self-defence". Mr Holder said: "The operation in which Osama bin Laden was killed was lawful. He was the head of al-Qa'ida, an organisation that had conducted the attacks of September 11. He admitted his involvement."

Mr Zawahiri is now America's most-wanted criminal. He is expected to stay as close to ground as possible. But analysts in the US noted that if he is to be accepted as the new head of al-Qa'ida and its splinter groups, he will have to find a way to assert his authority fairly soon and that will mean putting his head over the parapet. "Zawahiri will be very careful of his personal security, but he needs to consolidate his position; he needs to be out there," Daniel Byman, a counter-terrorism expert at the Brookings Institution in Washington, told Bloomberg. "He needs to meet and communicate."

A poll published by CBS and The New York Times showed an 11-point boost in Barack Obama's approval rating since news of Bin Laden's death.