Profile: Abdullah al-Senussi
Former Libyan intelligence chief Abdullah al-Senussi was one of the closest confidants of Libya's former leader Col Muammar Gaddafi.
Wanted by both the French authorities and the International Criminal Court, he fled Libya after Gaddafi's downfall and was arrested as he flew into Mauritania from Morocco in March on a false passport.
Mauritania's decision to return the former spy-chief to face Libyan justice raises the prospect of some of the closest secrets of the Gaddafi regime being laid bare.
Brig Gen Senussi rose from relatively lowly origins to become one of Libya's most powerful individuals. It was his marriage to Gaddafi's wife's sister in the 1970s that brought him into the ruling inner circle and he took up various roles including deputy chief of the external security organisation.
He was also said to be a close adviser to Gaddafi's son Saif al-Islam, according to leaked US embassy documents.
In June 2011 the International Criminal Court issued arrest warrants for Gen Senussi, along with Muammar and Saif Gaddafi, for crimes against humanity alleged to have been committed particularly against demonstrators in the eastern city of Benghazi at the start of the Libyan uprising in February 2011.
Observers considered him more of an executor than a mastermind behind the activities of the Gaddafi regime. He was nicknamed "the butcher" because of his reputation for brutal behaviour.Lockerbie knowledge
Gen Senussi has been accused of other human rights abuses, including his implication in the massacre in 1996 of more than 1,000 inmates at the Abu Salim prison in Tripoli.
He is alleged to have given the order to guards standing on grated ceilings above the inmates to fire down on them, after riots broke out over demands for better food and sanitary conditions.
He has been unable to travel abroad freely since he was convicted in absentia in 1999 by France for his role in the bombing of a French UTA passenger plane in 1989.
The plane blew up over the West African country of Niger, killing 170 people, many of them French.
Analysts also believe Gen Senussi has knowledge that could help the US and UK authorities establish the full facts about the 1988 Lockerbie bombing, in which 270 people died.
Recent investigations have cast doubt on the conviction of Abdelbaset Ali al-Megrahi, the only man brought to justice over the bombing. Gen Senussi is believed to have recruited Megrahi who died in May.
Both men were members of one of Libya's biggest tribes, the Magarha.Business interests
The former intelligence chief is also thought to have information about Libyans kidnapped and assassinated in Europe and elsewhere during Gaddafi's rule, and the financing of terrorist organisations, especially in Africa.
He has also been linked to a plot to assassinate Saudi Crown Prince Abdullah in 2003.
And he is said to have overseen the building of a secret nuclear plant in Libya's southern desert, the whereabouts of which has never been revealed.
He was already on a US treasury department blacklist of senior Libyan officials whose assets can be frozen if they are found inside US jurisdiction.
He is also said to have extensive business interests, like other members of Libya's political elite.
After the Libyan revolt broke out, Gen Senussi was accused of ordering the killing of protesters and recruiting foreign mercenaries fighting for Gaddafi.
Prosecutors and intelligence agencies believe his supposed intricate knowledge of the workings of the Gaddafi regime could shed light on of some of its most notorious acts.