Libya's prime minister Abdurrahim el-Keib says Muammar Gaddafi's infamous former spy chief Abdullah al-Senussi has been extradited by Mauritania Reuters

Mauritania said on Wednesday that it had extradited Muammar Gaddafi's infamous former spy chief, Abdullah al-Senussi, back to Libya, in a move that could shed fresh light on the 1988 Lockerbie bombing.

Government sources in Mauritania said Senussi had been sent to Tripoli "on the basis of guarantees given by the Libyan authorities". Senussi has been in custody in Mauritania since March, after slipping illegally into the country.

Officials in Tripoli could not immediately confirm Senussi's extradition, also reported by Mauritanian television. But foreign ministry spokesman Saad al-Shelmani said the country's transitional post-Gaddafi government welcomed the news.

He added: "We have been asking for this move for a very long time."

Senussi, Gaddafi's former director of military intelligence and a brutal enforcer, is one of the world's most wanted men. Libya, France and the international criminal court had all sought his extradition, with France seeking to question him in connection with the bombing of a French UTA passenger plane in 1989.

The ICC has indicted him for crimes against humanity in Libya.

Britain also has a strong interest in Senussi and is likely to seek to interview him in connection with the 1988 Lockerbie bombing, in which 270 died. At the time, Senussi headed Libya's external security organisation. He is said to have recruited Abdel-Basset al-Megrahi, the man convicted of the bombing. Megrahi died at his home in Libya in May.

The US is also seeking Senussi's arrest in connection with Lockerbie.

In an interview with the Guardian, Libya's prime minister, Abdurrahim el-Keib, said that as well as his alleged role in the Lockerbie bombing, Senussi knew the identity of the killer of PC Yvonne Fletcher, shot dead outside the Libyan embassy in 1984.

"He's the black box," Keib said, adding: "I guarantee he [Senussi] was almost directly or indirectly involved in most if not all of the crimes [of the former regime]. That doesn't mean others weren't involved. But he definitely knows who they were."

Senussi was married to Gaddafi's sister-in-law, and was at the Libyan dictator's side for over three decades. Leaked US diplomatic cables describe him as a trusted "senior regime figure", "who had played a role as minder of the more troublesome Gaddafi offspring".

They added: "Sanussi … is usually in physical proximity to the tent in which Gaddafi holds meetings with visiting foreign dignitaries and, according to members of Gaddafi's protocol office, personally oversees Gaddafi's close protection detail".

Libya's provisional government wants to try him in connection with numerous human rights abuses, including the massacre of 1,200 prisoners at the Abu Salim jail in 1996. During the 2011 Libyan civil war, he was blamed for orchestrating killings in the city of Benghazi and recruiting foreign mercenaries.

Senussi reportedly fled Tripoli as it fell to the rebels in August 2011, going first to Sirte and then heading southwards, home to his Megarha tribe before going to ground in Ghat. In October, when Gaddafi was killed, he crossed into Niger with a Tuareg escort. In November, according to reports, he was first in Mali then Mauritania and finally Morocco.

Senussi arrived in the Mauritanian capital, Nouakchott, from Casablanca on a scheduled Royal Air Maroc flight on 16 March. Mauritania said he was identified in a routine passport check but Arab sources claim French intelligence was involved in the operation.

The Moroccan magazine Telquel reported that he had been under surveillance for some time by Moroccan security services. Senussi remains a man of mystery and intrigue, but one thing is certain: Libyans want to see him in the dock.