Here's the riddle. If the Guardian, the New York Times and Channel 4's Dispatches can all find numerous journalists who worked at the News of the World who without exception insist that the newspaper routinely used private investigators to gather information by illegal means, why can't Scotland Yard find a single one who will tell them the story?

In their original inquiry into the phone-hacking affair, in 2006, detectives arrested the paper's royal correspondent, Clive Goodman, and charged him with listening to messages on the royal household's mobile phones. Goodman refused to answer questions.

Scotland Yard then interviewed not one single other journalist, editor or manager from the paper. Detectives took this decision despite holding evidence that – we now know – clearly identified other News of the World journalists who were involved in handling illegally intercepted voicemail.

In their recent inquiry, which ended fruitlessly last week, they attempted to interview only three journalists, all of whom were identified for them by news organisations.

They approached those three not as witnesses but as suspects, warning them that anything they said could be used to prosecute them: two gave interviews in which they declined to answer questions; the third challenged them to arrest him in handcuffs, and so they never even spoke to him.

Scotland Yard appears to be playing a dangerous game. The latest inquiry was the third time police have looked at the phone-hacking saga and the third time they have come up empty: neither the prime minister's media adviser and former News of the World editor, Andy Coulson, nor anybody else from the paper has any case to answer, they say.

The police will say they are playing by the rules. The danger is that some of the public figures who believe their voicemail was intercepted will bring out the evidence that police have failed to find, in which case Scotland Yard is going to be accused of being involved in a cover-up. A kind of do-it-yourself policing is emerging.

These public figures span a spectrum from the most powerful in the land – Tony Blair and his family, Gordon Brown and his wife, Sarah, through to global celebrities such as Sienna Miller, to classic tabloid targets including the late Jade Goody's trustees.

Some have been told that police do have evidence that Mulcaire was targeting them, although Scotland Yard refuse to reveal it without a court order. Some have been told there is no evidence about them. Some have complained that they have received misleading answers. And several dozen of them have recently received letters admitting that they may have been misinformed.

A growing number is now on the way to court. And there is the real danger.

This whole affair broke open last year because one of the few victims named in court, Gordon Taylor of the Professional Footballers' Association, sued the News of the World. The judge ordered police to hand over relevant evidence, which implicated three journalists, and the paper paid £1m to Taylor and two others on condition that they kept it quiet. Which they did – until the Guardian revealed the secret settlement last year.

Now, more and more public figures are following Gordon Taylor to court, where they will ask judges to order disclosure of relevant material from two caches of evidence, which police have been sitting on for more than four years. The first cache is the data they gathered from phone companies in the first six months of their inquiry, in 2006. It was this data which enabled them to tell the Crown Prosecution Service at the time that "a vast number of unique voicemail numbers belonging to high-profile individuals have been identified as being accessed without authority". The CPS said this revealed "a vast array of offending behaviour".

Police then arrested Goodman and Mulcaire, at which point, from the investigator, they obtained their second cache of evidence: a mass of computer records, hand-written notes, audio tapes, including 3,000 mobile phone numbers, as well as invoices on which Mulcaire had identified the public figures he was targeting.

The original investigation failed to pursue the "vast array of offending" revealed by the phone data and simply filed the cache of material seized from Mulcaire without even searching it.

When the Guardian revived the story last year, an assistant commissioner, John Yates, spent less than 24 hours "establishing the facts" of the case and announced his conclusion without knowing the contents of the seized material. The most recent inquiry, which ended last week, was tasked to consider only "new" evidence and so once again ignored the material which the police have held for more than four years.

Quite why Scotland Yard should behave like this remains unproved – another riddle waiting to be solved.