There is often a moment in old-fashioned detective fiction when someone decides to "call in the Yard". Bumbling local cops are sidelined as worldly-wise detectives arrive from London, spotting missed clues and identifying suspects. Hence the sense of déjà vu I experienced a couple of days ago when there was a "breakthrough" in the Madeleine McCann case. Scotland Yard has launched a new investigation, detectives are supposedly about to "swoop" and arrests could be made "within weeks".

The popular press has always treated this abduction as a completely irresistible mystery. Since she disappeared in Portugal in 2007, Madeleine has been "spotted" I don't know how many times, while one "suspect" after another has been dismissed from the inquiry. There is a difference this time, which is that Madeleine's parents, Kate and Gerry McCann, are being treated with kid gloves. I suspect this has more to do with their successful libel action against Express newspapers, in which they were awarded damages of £550,000, than a sudden outbreak of compassion and decency.

In other respects, the announcement produced the usual round of febrile speculation, despite an attempt by Scotland Yard to manage expectations. All we know for certain is that detectives have identified 38 "persons of interest", including known sex offenders, during a lengthy review of the case. Twelve are UK nationals whom the police believe were in Portugal when Madeleine went missing.

But a "person of interest" is not the same as a prime suspect. Logic dictates that most, if not all, of the 38 will turn out to have nothing to do with the case, while the police have been careful to talk about no more than the "possibility" that Madeleine is alive. The review which preceded the inquiry involved 30,500 documents, material collected by both British and Portuguese police forces and the findings of seven private-detective agencies. The fact that so much material has been amassed without producing hard evidence shows how difficult this case is; vital leads are usually generated within hours of a crime, and detectives are dealing with events that took place six years ago.

I'm sure Kate and Gerry McCann, whom I've met on several occasions, are desperately hoping they will finally discover what happened to their daughter. No one who heard their testimony to the Leveson inquiry could doubt how much they've suffered because of the loss of their child, but I can't help wondering about the timing of the decision. It comes after a dreadful few weeks for the Metropolitan Police, whose reputation seems to become ever more battered with every news bulletin.

Already the McCann inquiry has produced headlines suggesting that Scotland Yard will succeed where their Portuguese colleagues failed, but it's a risky strategy. Meanwhile, the McCanns will have to brace themselves for weeks of speculation – and what could be a big test of the conduct of the popular press after the drubbing it received in the Leveson report.

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