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John Rentoul: When not knowing is not good enough

MI5 and MI6 sources say there may be 15 cases of UK complicity in torture. An inquiry is welcome but may not get many answers

Baroness Scotland, the Attorney General, is an oddity. The description is not mine, but hers. She once explained, in a book called Why I Am Still a Catholic: "I know that whatever I have done as a minister, and will do, it is by the grace of God. I'm not in control of the agenda." She added: "I realise that putting it in such terms is unusual for a politician, but as a black socialist Catholic in government I am already an oddity anyway."

Well, by the grace of God, there will now be a police investigation into the allegation that British agents encouraged the torture of Binyam Mohamed, an Ethiopian, in Pakistan and Morocco in 2002. Lady Scotland decided last week that MI5 had a case to answer, so asked the Metropolitan Police to undertake its first criminal inquiry into the actions of one of the intelligence services.

This is, of course, a good thing. On balance, we should be in favour of inquiries, and attempts to find out the truth generally. But we should be cautious in our expectations. People who call most loudly for inquiries often do not want to know what happened – they think they know. They want their beliefs confirmed.

Those who are most sure that Tony Blair lied about Iraq are most vocal in demanding (another) inquiry to prove it. When it concludes that he did not, although I hope that its main focus will be on more constructive matters, they will demand another inquiry to find out how the intelligence services, the Attorney General, the Archbishop of Canterbury and the Saudi royal family conspired to conceal the truth.

So it is that some of those most eager to put ministers in the dock for the torture of Binyam Mohamed have already decided that this government is, in the vogue word, "complicit". The chances are that they won't be satisfied by the outcome of the police investigation. But in this case they may have a point. "I don't think we should kid ourselves into thinking that a police investigation will necessarily close this issue," said Peter Goldsmith, the former attorney general, on the Today programme on Friday.

Some of the outlines of the case for defence were sketched yesterday in the Daily Telegraph, one of the more spook-friendly newspapers. The security services, it would seem, would like a further 15 doubtful cases taken into consideration. They say that (a) they didn't realise what the Americans were up to, and (b) that, as soon as they suspected, they sent a memo to all agents reminding them about the Geneva Conventions. The trouble is that this was as early as January 2002, and it implied that they should not try to stop the mistreatment of prisoners held by the US: "Given that they are not within our custody or control, the law does not require you to intervene to prevent this."

Of course, we should not assume that MI5 officers knew or even should have known that Mohamed was being badly treated when they interviewed him in Pakistan or supplied questions to his interviewers in Morocco. And I suspect that Lord Goldsmith is right that we shall never find out. But that does not mean that the police investigation into the Mohamed case, or a wider inquiry into other cases, is a waste of money.

First, accountability is important. So far, David Miliband has been criticised unfairly. His role has been to supply Mohamed's lawyers with intelligence documents that help his case, and to allow him to return to Britain, where he once lived, on his release from Guantanamo Bay. Miliband wasn't Foreign Secretary in 2002: at the time, Jack Straw was responsible for dealing with the Americans and other governments. And it was David Blunkett, the Home Secretary, who was directly responsible for MI5. I understand that Blunkett is irritated that neither his successor, Jacqui Smith, nor Lady Scotland told him that he might be the subject of a police investigation.

Not that I believe for a moment, if Mohamed was tortured, and if MI5 agents were "in the next room" at the time, that Blunkett would have known about it. The person who might have known is Jonathan Evans, who was head of international counter-terrorism at MI5 at the time, and who is now its Director General. But, as Lord Goldsmith suggested, we would be kidding ourselves if we thought it would be easy to establish either wrongdoing or responsibility for it.

However, an inquiry into the Mohamed and other cases would still be valuable. Not so much for establishing what happened up to seven years ago, but for sending a shock through the system to make it less likely in future that anyone in the British security services thinks it a good idea to turn a blind eye to torture.

Because the important lesson of the Mohamed case is that torture is counter-productive. Some of his story of how he ended up in an Afghan training camp on a false British passport seems unconvincing. But it is believable that he would be prepared to admit to anything – in his case a "dirty bomb" plot that was put to him by his questioners – in order to stop the pain. If Mohamed ever had any useful intelligence about jihadi terrorist plans, its value was destroyed by the way he was treated.

Torture is morally abhorrent. But those are just words. What may make a difference, and what an inquiry may help to establish, is that it doesn't work.