On Saturday morning, the police arrested four journalists who have worked for Rupert Murdoch. For a while, it looked as though these were yet more arrests of people related to the News of the World but then it became clear that this was something much more significant.

This may be the moment when the scandal that closed the NoW finally started to pose a potential threat to at least one of Murdoch's three other UK newspaper titles: the Sun, the Times and the Sunday Times.

The four men arrested on Saturday are not linked to the NoW. They come from the Sun, from the top of the tree – the current head of news and his crime editor, the former managing editor and deputy editor.

Nothing is certain. No one has been convicted of anything. The four who were arrested on Saturday – like the 25 others before them – have not even been charged with any offence. But behind the scenes, something very significant has changed at News International.

Under enormous legal and political pressure, Murdoch has ordered that the police be given everything they need. Whereas Scotland Yard began their inquiry a year ago with nothing much more than the heap of scruffy paperwork seized from the NoW's private investigator, Glenn Mulcaire, Murdoch's Management and Standards Committee has now handed them what may be the largest cache of evidence ever gathered by a police operation in this country, including the material that led to Saturday's arrests.

They have access to a mass of internal paperwork – invoices, reporters' expense claims, accounts, bank records, phone records. And technicians have retrieved an enormous reservoir of material from News International's central computer servers, including one particularly vast collection that may yet prove to be the stick that breaks the media mogul's back. It is known as Data Pool 3.

It contains several hundred million emails sent and received over the years by employees of the News of the World – and of the three other Murdoch titles. Data Pool 3 is so big that the police are not even attempting to read every message. Instead, there are two teams searching it for key words: a detective sergeant with five detective constables from Scotland Yard working secretly on criminal leads; and 32 civilians working for the Management and Standards Committee, providing information for the civil actions brought by public figures and for the Leveson inquiry and passing relevant material to police.

For News International, Data Pool 3 is a nightmare. Firstly, no one know what is in there. All they can do is wait and see how bad it gets.

Second, the police clearly believe it may yield new evidence of the crimes they set out to investigate – the "blagging" of confidential data from phone companies, banks, tax offices etc; the interception of voicemails and emails; the payment of bribes to police officers.

Third – and most nightmarish – Data Pool 3 could yield evidence of attempts to destroy evidence the high court and police were seeking. Data Pool 3 itself was apparently deliberately deleted from News International's servers.

If proved, such conduct would be serious because it could see the courts imposing long prison sentences; and because it could have been sanctioned by senior employees and directors.

The Guardian last July revealed police suspicions that a huge number of emails had been deliberately destroyed. Since then, high court hearings have disclosed more detail. Late in 2009, News International decided to delete old email from their servers. This appears to have been a simple piece of electronic housekeeping. However, the plan was not executed.

During the summer of 2010, the actor Sienna Miller decided to sue the NoW for hacking into her voicemail. At the same time, according to evidence in the high court civil claim, internal emails were being sent urging that the deletion plan be executed. Still, it was not.

On 6 September 2010, Sienna Miller's solicitor, Mark Thomson of Atkins Thomson, wrote to News International asking them to "preserve all the documents in your possession relating to our client's private life".

On 9 September, an internal message pressed for the emails to be deleted "urgently". As Mr Justice Vos explained in a judgment last month: "Only three days after the solicitors for Sienna Miller had written their letter before action, asking specifically that the company should retain any emails concerned with the claim, what happened was that a previously conceived plan to delete emails was put into effect at the behest of senior management."

In December 2010, the NoW's Scottish editor, Bob Bird, told the trial of Tommy Sheridan in Glasgow that the email archive had been lost en route to Mumbai. Also in December, News International's solicitor, Julian Pike from Farrer and Co, provided the high court with a statement claiming they were unable to retrieve emails more than six months old.

On 7 January 2011, News International gained access to the evidence that had been assembled by Sienna Miller's lawyers. On 12 January, the company issued detailed instructions for the secure retention of relevant data. Later that month, News International handed three old emails to Scotland Yard, triggering the new police inquiry. In the same month, a second significant deletion is believed to have happened. By this time, the entire contents of Data Pool 3 had been deleted.

However, under pressure from the lawyers involved in the high court civil actions, News International were compelled to allow technical experts to examine their servers.

On 23 March 2011, Pike formally apologised to the high court and acknowledged that News International could retrieve emails as far back as 2005 and that none had been lost en route to Mumbai. He said he had been misinformed.

In October, technicians started to restore the millions of deleted emails. By December, the entire contents of Data Pool 3 had been recovered. The implications are considerable.

On Saturday, as police searched parts of the Sun office, a press release from News Corp referred discreetly to an "internal investigation into our three remaining titles." The Times is already under pressure from an allegation that a reporter hacked into a target's email to obtain a story. In an unexplained line in his statement to the Leveson inquiry, the Sunday Times editor, John Witherow, said "a freelance journalist/researcher who has done occasional work for the paper was arrested on suspicion of breaching the Fraud Act. The police investigation is still continuing."

Whether more of News International's UK titles are dragged into the police inquiry remains to be seen. The threat is there: it may or may not materialise. Similarly, it is not yet clear whether police will find evidence that senior employees and directors did order the destruction of evidence. Equally important, the police may find evidence of more victims who may want to launch more legal actions.

At the outer reaches of possibility, police may find evidence of illegal activity by other private investigators, which could conceivably lead them to other news organisations who also hired them. Since Saturday morning, nothing is certain.