Libya's former spymaster Abdullah Al-Senussi, who was arrested Saturday in Mauritania, was considered the most powerful pillar of Muammar Gaddafi's repressive regime.
Senussi, a brother-in-law of the slain dictator who is wanted by the International Criminal Court, was captured overnight at Nouakchott airport, a Mauritanian security official told AFP.
After months on the run, Senussi was arrested as he tried to enter Mauritania with a "fake Malian passport" on a regular flight from Morocco, the source added.
The ICC says Senussi played a "crucial" role in crushing the popular revolt that eventually ousted the Gaddafi regime late last year.
In November, Libya's new rulers had announced the capture by rebels of the spy chief who is in his early 60s, but the heavy-set man with a thick jawline and bushy black eyebrows remained elusive.
At the time, National Transitional Council (NTC) officials as well as military commanders said Senussi was arrested at his sister's home in the southern Libya region of Al-Guira where he had apparently been hiding.
That announcement came day after the arrest of Seif Al-Islam, the 39-year-old son and heir apparent of Gaddafi, who himself was seized and slain on 20 October.
Last June, the ICC had issued arrest warrants for the trio on charges of crimes against humanity over the violent repression of anti-regime protests in the North African state.
There was no immediate comment from NTC officials of Senussi's reported arrest in Mauritania.
Senussi is one of the last members of Gaddafi's inner circle to have been arrested. Others still at large include Gaddafi's son Saadi who has taken refuge in Niger.
Senussi was the national head of the military intelligence, which the ICC described as "one of the most powerful and efficient organs of repression of Muammar Gaddafi's regime."
The ICC says Senussi was an "indirect perpetrator of crimes against humanity of murder and persecution based on political grounds" committed in Benghazi from 15 February until at least 20 February this year.
"(Senussi), once instructed by Muammar Gaddafi to implement the plan of deterring and quelling civilian demonstrations against the regime in Benghazi... directly instructed the troops to attack civilians demonstrating in the city."
According to The Hague-based court, Senussi was "in a position to trigger the actions of the armed forces and ensure compliance with such orders, and therefore, the commission of crimes by any replaceable direct perpetrator."
Long considered the slain dictator's right-hand man, Senussi remained faithful to the end to the man who ruled Libya with an iron-fist for 42 years.
On 21 August, the day rebels stormed Tripoli, Senussi made a rare appearance at the Rixos Hotel, headquarters of the foreign media in the Libyan capital, to address journalists.
He ranted against NATO's bombing campaign and charged that the military alliance, which bombed his Tripoli home a few days earlier, worked hand in hand with Western intelligence and Al-Qaeda "to destroy Libya."
"Libya will not be ruled by bands of terrorists," Senussi said at the time.
In 1999, a Paris court sentenced Senussi in absentia to life for his alleged involvement in an attack on a French UTA airliner a decade earlier that killed 170 people.
Senussi could also be held accountable for the Abu Salim prison massacre of 1996 when more than 1,000 detainees were gunned down.