Mauritania has sent Colonel Gaddafi's spy chief, Abdullah al Senoussi, to Libya, according to reports.
It comes nearly five months after he was arrested for entering the country illegally.
An official source, speaking on condition of anonymity, said that al Senoussi had "effectively left Mauritania".
A delegation from Libya, including the defence minister and army chief of staff, were in Mauritania's capital, Nouakchott, on Tuesday for a visit which several official sources said was in connection with the extradition.
"He was extradited to Libya on the basis of guarantees given by Libyan authorities," a government source told Reuters, without revealing any details of the guarantees.
Al Senuossi was arrested five months ago at the airport in the capital Nouakchott after arriving from Morocco. He was reportedly travelling on a fake Malian passport.
The International Criminal Court in the Hague issued an arrest warrant in June 2011 for both al Senuossi and Gaddafi's son, Saif al Islam, who was captured disguised as a Bedouin in the Sahara desert in November.
Security sources in Niger and Mali said in October 2011 that al Senoussi and several of his men had passed through their territory. A month later, Libya's new government announced his arrest but no pictures of him were released.
Al Senuossi is the target of another international arrest warrant after a Paris court sentenced him in absentia to life in prison for involvement in the bombing of a French UTA airliner over Niger in September 1989.
The plane was carrying 170 people from Brazzaville to Paris, via N'Djamena.
That attack, along with the bombing of a Pan Am jumbo jet over Lockerbie, Scotland, in December 1988 in which 270 people were killed, led to a UN-mandated air blockade of Libya in 1992.