THE capture yesterday of former Libyan leader Muammar Gaddafi's intelligence chief could provide the key to unlocking the truth about the Lockerbie bombing.

Scottish prosecutors hope the arrest of ex-spy chief Abdullah al-Senussi means they will finally be able to settle the unanswered questions about the blowing up of Pan Am Flight 103 on December 21, 1988, which killed 270 people.

The Libyan authorities yesterday confirmed that Senussi was detained at Nouakchott airport, in Mauritania, while travelling on a fake Malian passport.

The 63-year-old, a trusted aide and Gaddafi's brother-in-law, belongs to the Magarha tribe, believed to be the second-biggest in Libya. Abdelbaset Ali Mohmed al-Megrahi, the only man convicted and jailed for the bombing, belongs to the same tribe.

It is understood that Senussi, who was wanted in connection with the Lockerbie atrocity, could now be questioned by Scottish police.

A spokesman for the Crown Office said: "The investigation into the involvement of others with Megrahi in the Lockerbie bombing remains open and the Crown will work with Dumfries and Galloway Constabulary and US authorities to pursue available lines of enquiry."

Senussi fled Libya when Gaddafi was ousted and killed last year, after months of fighting.

At the time, the International Criminal Court (ICC) issued a warrant for his arrest for crimes against humanity.

He was seized by security officials yesterday when he arrived in Nouakchott on a flight from the Moroccan city of Casablanca.

France said the arrest was a joint operation between French and Mauritanian authorities, and President Nicolas Sarkozy said he would request Senussi's extradition.

A French court convicted Senussi – in absentia – of involvement in a 1989 attack on a French airliner in the West African country of Niger that killed 170 people, and sentenced him to life in prison.

But Libyan authorities are also demanding his extradition.

Mauritania has not signed the ICC's statute, and it is unclear what the country will do with Senussi.

Megrahi was convicted of the Lockerbie bombing and sentenced to life imprisonment in 2001. He was freed on compassionate grounds from Greenock jail in August 2009, suffering from prostate cancer, after doctors advised he had around three months to live.

US cables published by WikiLeaks said Senussi asked US diplomats to help smooth the way for Megrahi's release but they demurred.

British Foreign Secretary William Hague welcomed yesterday's arrest and said Senussi must face trial for his part in the repression in Libya.

Hague said: "Like others who have sought to evade justice, he must now be held to account for his actions and face trial on the charges brought against him.

"I congratulate the Mauritanian authorities for detaining him and encourage all involved to co-operate fully with the ICC (International Criminal Court) under UN Security Council Resolution 1970.

"The detention of the final Libya ICC indictee now offers the Libyan authorities the opportunity to focus fully on rebuilding their country and continuing the transition to democracy."

Nicknamed "the butcher", Senussi was one of the last significant members of Gaddafi's regime who was still at large.

He was indicted by the ICC along with Gaddafi and the leader's son Saif al-Islam on June 27, 2011.

Saif was captured in November in southern Libya and has been held by former rebels ever since. The ICC wants him tried in The Hague but the Libyan authorities say he will receive a fair trial at home.

Senussi is believed to have been responsible for purges of opponents within the regime in the 1980s and 90s, and for the deaths of 1200 political prisoners at Tripoli's Abu Salim prison in 1996.

He kept a low public profile during last year's uprising, but reportedly played a key role in attempts to crush the revolt in the eastern city of Benghazi last February.

There have been repeated reports of his death and capture – which were later proved false.

In October, officials in Niger said he had fled through the countryinto Mali, but a month later the new Libyan authorities said he had been arrested in the southern Libyan region of Sabha.

Further reports of his capture came in November and December last year, but officials were unable to provide evidence.