THE phrase extraordinary rendition is a disturbing example of the use of official jargon to conceal an unacceptable truth.

It is used to describe the transfer of terrorist suspects for interrogation by the CIA in countries where torture is not illegal.

The investigation into rendition flights ordered by Lord Advocate Frank Mulholland is a long-overdue step towards discovering the truth of the involvement of the UK Government in this process and the extent to which Scottish airports were used and on what basis.

We have known for several years Scottish airports are likely to have been used for re-fuelling or overnight stopovers for flights linked to the CIA. Researchers examining the use of such planes for prisoner transfers now say they have "conclusive" evidence of landings at Scottish airports. In addition to the use of Prestwick, Glasgow and Edinburgh as a stop-off on journeys between the US and Middle East, which had already been identified, they have discovered five flights which landed at Wick, a further five at Inverness and three at Aberdeen, whose flight paths put them in the "suspicious" category. It is not known if the planes had prisoners on board but one aircraft which landed at Wick in 2004 has been logged flying to secret prison and torture destinations according to the researchers on the Rendition Project, which has mapped thousands of rendition flights.

It is evident from this database that the support provided by the UK for the CIA's global rendition programme was more substantial than previously realised. It remains unclear whether this was with the approval of the UK Government or whether it deliberately chose not to ask questions about the purpose of unusual flight routes.

After the use of UK airports by planes linked to rendition operations was revealed in 2005, Jack Straw, the then Foreign Secretary, said the US would require the Government's permission to render a detainee through Britain. In 2011, he said: "We were opposed to any use of torture or similar methods. Not only did we not agree with it, we were not complicit in it and nor did we turn a blind eye to it."

Yet it is difficult to avoid the suspicion that, if a blind eye was not turned, there were plenty of wilfully deaf ears and unasked questions. A leaked memo revealed it was not known whether those captured by British forces in Iran or Afghanistan were subsequently sent to interrogation centres.

We know that the US operated rendition flights because the then Secretary of State Condoleezza Rice said it had been policy for many years, although she denied that torture was used. We now know that several Scottish airports were used, probably for refuelling. Although this is substantially a UK Government matter, if Scotland is not be complicit in the rendition of detainees to countries where systematic torture is known to be used, it is essential to establish where these planes were going and why, who was on board and on what basis their landing was authorised.

A previous police inquiry found insufficient evidence for a criminal investigation. That a new one has been instigated as a result of the additional information is a welcome recognition of the need not only to condemn torture but to prevent it and to expose the abhorrent practice.