PROSECUTORS working on the fresh inquiry into the Lockerbie bombing will "pursue all available lines of inquiry" with Libyan authorities, following the extradition of Colonel Gaddafi's intelligence chief back to his home country.

Abdulla al-Senussi, former head of Libya's domestic intelligence service, has been released by the Islamic Republic of Mauritania, on the west coast of Africa, where he sought refuge following the downfall of the Gaddafi regime last year.

He is thought to be key to any inquiry into the 1988 atrocity, in which 270 people died, and the murder of police woman Yvonne Fletcher in London four years earlier, which is being probed by Scotland Yard.

Lord Advocate Frank Mulholland travelled to Libya in April to meet Prime Minister Abdurrahim el-Keib and pave the way for the new Lockerbie inquiry, announced last autumn.

A statement from the Crown Office said: "We note the position in relation to the extradition of Senussi to Libya and we will continue to liaise with our colleagues in the Foreign and Commonwealth Office, as well as the Libyan authorities, to pursue all available lines of inquiry."

It would not be drawn on whether active steps were being made to interview Senussi, who has been accused of crimes against humanity – including murder and persecution – by the International Criminal Court.

The ICC has also indicted him on alleged involvement in "fraud offences including embezzling public funds and misuse of power for personal benefit". France has also battled to take him into custody over a 1989 airliner bombing over Niger in which 54 of its nationals died.

Senussi reportedly arrived in Tripoli last night after guarantees were given by the Libyan authorities. He was first arrested six months ago after arriving with a false Malian passport on a flight into the Mauritanian capital Nouakchott from Morocco.

Libya has pledged that the former intelligence chief will get a fair trial amid concerns over the treatment of those involved in the former regime.

Prime Minister el-Keib said yesterday: "Abdullah al-Senussi will have a fair trial according to international standards for human rights."

Also in Libya, Gaddafi's son, Saif al-Islam will go on trial this month.