On the morning of 8 August 2006 officers of the Metropolitan Police raided the offices and home in Surrey of the private investigator Glenn Mulcaire and gathered up all the materials they could find which might be relevant to their ten-month-old investigation into the illegal hacking of mobile phone voicemails. The haul — notebooks, loose papers, files, disks of various kinds, computer records — was put into bin bags, filling two or three of them.

Five years on, what was done with the contents of those bin bags — in other words, who looked at what, and in particular, when — is a matter of great interest, or concern.

Since the police had gone to the trouble of seizing the materials, you might think that they would have been curious to see what they revealed — after all, they were well on their way to building a case against Mulcaire for hacking members of the royal household, even Princes William and Harry. What else might they find? It is easy to imagine detectives rolling up their sleeves and eagerly getting to work.

More and more, however, it looks as though they barely opened the bin bags, indeed only this week it was revealed for the first time that the haul from Mulcaire included more than 100 recordings likely to be of voicemail messages on tapes and minidiscs. Everything suggests that this was a relatively new discovery for the Metropolitan Police, and it was only the latest of its kind.

In fact it seems now that detectives only started looking seriously at the contents of the bin bags towards the end of 2009 — three years on. Acting deputy assistant commissioner John Yates confirmed to MPs this April that the first efforts to transfer the information — a mass of numbers, codes, names and addresses — to a computer database did not begin until after the Guardian revived interest in hacking in July of that year. By implication the police only began to grasp the enormity of what they had in their possession around the middle of 2010.

Why would detectives seize all that documentation and then do nothing with it for years? Here are three possible explanations.

1. The police story
The implication of the Met’s account of events is that senior officers believed there was no justification for an exhaustive exploration of the bin bags. There was already a good case against Mulcaire and the News of the World royal editor Clive Goodman, gathered by electronic surveillance and by liaison with the mobile phone companies; officers looked in the bags for anything that would confirm this, back up the case and spare them from revealing their secret techniques in court. Once they had found what they wanted they packed up the rest and forgot about it.

The investigation ended with both suspects convicted and sent to prison, sending a clear warning to anyone tempted to hack voicemails. Any further rummaging around in the bags would have been a waste of public money.

2. The New York Times story
A second hypothesis, in various forms, has had wide currency as a suspicion, and was most explicitly expressed in reporting by the New York Times.

The paper said that “several [police] investigators said in interviews that Scotland Yard was reluctant to conduct a wider inquiry in part because of its close relationship with News of the World”.

It noted that the police decision to notify relatively few of the people whose messages may have been illegally accessed had the effect of shielding Rupert Murdoch’s News International from civil actions and public criticism.

The NYT wrote:

Within days of the raids [on Mulcaire and Goodman], several senior detectives said they began feeling internal pressure. One senior investigator said he was approached by Chris Webb, from the department’s press office, who was ‘waving his arms up in the air, saying, “Wait a minute — let’s talk about this.” ’ The investigator, who has since left Scotland Yard, added that Webb stressed the department’s ‘long-term relationship with News International.’ The investigator recalled becoming furious at the suggestion, responding, ‘There’s illegality here, and we’ll pursue it like we do any other case.’ In a statement, Webb said: ‘I cannot recall these events. Police officers make operational decisions, not press officers. That is the policy of the Metropolitan Police Service and the policy that I and all police press officers follow.’

3. The wrong coppers version
This third possibility, not widely canvassed, depends on two facts. The first is that the part of the Met which was assigned to investigate phone hacking was the anti-terrorist branch. This was because the royal family was involved, and that branch happens also to have responsibility for royal protection. The second fact is that, for good reasons or bad, the Met seems to have convinced itself that the only convictions worth getting for voicemail hacking required them to prove something extremely difficult to prove — that the hacker heard the message before the intended recipient.

Could it be that anti-terrorist officers, who are used to tackling highly complex bombing conspiracies in liaison with international intelligence agencies and top government officials, felt that rooting through bin bags for evidence of celebrity eavesdropping was not a fit use of their time? Could it be that, having persuaded themselves that the only way to get worthwhile convictions was by high-powered “technical” means, they were not interested in a laborious, document-based approach that might end with nothing more than fines? Could it be that, having proved a case involving the royal household, they were not interested in offences against lesser mortals?  Could it be, in short, that the bin bags had ended up in the possession of the wrong sort of cops?

Whether any of these is the correct explanation, or whether it is something entirely different, we do not know. What is more, no matter what happens in the civil or criminal courts, we will never know, unless there is a full public inquiry into these matters. Even the Met has every interest in getting the truth on the record, because until that happens the suspicions will remain.

Brian Cathcart teaches journalism at Kingston University London and tweets at @BrianCathcart