Senior al-Qaeda commander Saif al-Adel has been arrested at Cairo airport, Egyptian security officials say.
He was detained after arriving in the capital on a flight from Pakistan.
Officials told state media they knew he was intending to hand himself in, and that his real name, Mohammed Ibrahim Makkawi, was on the passenger list.
The former Egyptian army colonel was once Osama Bin Laden’s security chief and is wanted by the US for the 1998 embassy bombings in East Africa.
He was also suspected of training Somali fighters who killed 18 US servicemen in Mogadishu in 1993, and some of the 11 September 2001 hijackers.
Saif al-Adel is on the FBI’s most-wanted list and the US has offered a $5m reward for information leading to his capture or death.
Security officials told the Egyptian state news agency Mena that Saif al-Adel was detained at Cairo International Airport as he arrived on Wednesday, after flying to Egypt from Pakistan via Dubai.
The officials said they had received information about his plans to return to Egypt and hand himself over the authorities.
All flights from Asia were monitored as he was expected to come from either Afghanistan or Pakistan, and eventually his name was spotted on the passenger list of an Emirates Airline flight, they added.
Saif al-Adel has been handed over to the Higher State Security Prosecution for interrogation. Mena did not say where is being held.
However, the Associated Press reports that a man identifying himself as Mohammed Ibrahim Makkawi told journalists at the airport that he was not the senior al-Qaeda leader known as Saif al-Adel.
Saif al-Adel, a former Special Forces commander, who is in his 50s, first travelled to Afghanistan in the 1980s to fight Soviet forces with the mujahedeen.
He has been wanted by the Egyptian authorities since 1987, when he was accused of trying to establish a military wing of the Egyptian Islamist group al-Jihad, and trying to overthrow the government.
Saif al-Adel later joined al-Qaeda and became Osama Bin Laden’s security chief. He assumed many of military commander Mohammed Atef’s duties after his death in a US air strike in November 2001.
Following the US-led invasion of Afghanistan in 2001, Saif al-Adel is believed to have fled to Iran with Saad Bin Laden, a son of the late al-Qaeda leader.
They were allegedly then held under house arrest by the Revolutionary Guards, although Iran never acknowledged their presence.
Several letters and internet statements bearing Saif al-Adel’s name or aliases were subsequently released, leading analysts to believe he was still in contact with al-Qaeda’s leaders in the region.
Recent reports said Saif al-Adel might have been released and made his way to northern Pakistan, along with Saad Bin Laden.
There was also speculation that he was appointed temporary leader of al-Qaeda after Osama Bin Laden was killed in a raid by US Special Forces on a compound in the north-western Pakistani city of Abbottabad last May.