|Al-Adel is wanted in connection with the 1998 bombings of US embassies in Africa
Saif al-Adel, a senior al-Qaeda operative believed to have briefly taken over the network's leadership after Osama bin Laden's death, has reportedly been arrested in Egypt.
Security officials said al-Adel, who is on the FBI's most-wanted list with a reward of up to $5m for his capture, was detained at Cairo airport on Wednesday.
While Egyptian officials said they had identified the man held as al-Adel, other sources questioned this account.
Officials said he had arrived on an Emirates Airline flight from Pakistan with a stop over in Dubai.
Security officials said al-Adel, a former Egyptian army officer born in 1963, was arrested after presenting a travel document at customs and taken into custody by the National Security apparatus. They said he had entered the country using the known alibi Muhamad Ibrahim Makkawi.
Omar Ashour, who has been researching al-Qaeda and Islamist movements, said al-Adel and Mekkawi are two different persons, with different different ties to al-Qaeda. He told Al Jazeera that confusion over the identities came from similarity in their profiles, including that they had both served in the Egyptian army as officers.
The FBI said on its website that al-Adel "is thought to be affiliated with the Egyptian Islamic Jihad (EIJ), and is believed to be a high-ranking member of the al-Qaeda organisation".
He was wanted since 1994 in connection with the August 7, 1998, bombings of the United States Embassies in Dar es-Salaam, Tanzania, and Nairobi, Kenya.
Al-Adel was reported to have taken over the leadership of al-Qaeda before the appointment of another Egyptian, Bin Laden's longtime deputy Ayman al-Zawahiri.
An airport source speaking to Al Jazeera said al-Adel told reporters at the airport that he left al-Qaeda long time ago.
He travelled to Afghanistan in the 1980s to fight against the Soviet invasion of Afghanistan. He reportedly left the country after the September 11, 2001 attacks in the US, and was later detained in Iran.