Dark rumours were swirling as long ago as last spring, when Rupert and James Murdoch paid three unprecedented visits to Wapping in the space of a month.

Not only were father and son considering closing the scandal-racked News of the World, went the chatter on the Wapping grapevine, but its sister paper, the Sun, was also in the line of fire. Back then the fears seemed outlandish, born of the febrile atmosphere around the Sunday title that was to bring about its demise last July. There was little evidence the toxic allegations of malpractice would spread to the daily tabloid.

But what seemed incredible last year is now being discussed openly in Fleet Street following the arrests of five Sun employees on suspicion of bribing public officials. They follow the arrest of four other current and former Sun executives and journalists in January – and the separate arrest of a reporter on the paper the previous November – on similar suspicions.

The drip, drip nature of the arrests is in danger of becoming a flood. What started as a separate, but minor, line of investigation for the Metropolitan police team predominantly charged with examining allegations of phone hacking now threatens to become an epic bribery scandal of equal gravity.

A total of 21 people have now been arrested in the bribery probe, Operation Elveden, including three police officers, though no one has yet been charged. Those arrested include Rebekah Brooks, the former chief executive of Murdoch's News International, the company that owns the Sun, and ex-News of the World editor Andy Coulson, who went on to become prime minister David Cameron's communications chief.

That the arrests are linked to alleged bribes paid not just to police officers but prison staff and Ministry of Defence officials, confirms Scotland Yard is throwing its net wider as it seeks to root out corruption. The arrest of an MoD official may invite speculation that the Official Secrets Act could have been breached.

The investigation is expected to examine allegations of payments from other non-Murdoch titles to officials soon. But it is the Sun that is currently feeling the heat. Yesterday the paper attempted to put a brave face on things.

"I'm as shocked as anyone by today's arrests but am determined to lead the Sun through these difficult times," said the paper's editor, Dominic Mohan. "I have a brilliant staff and we have a duty to serve our readers and will continue to do that. Our focus is on putting out Monday's newspaper."

The allegations will revive concerns that News International is too close to the police, including officers at the most senior levels. Andy Hayman, the Met's counter-terrorism expert who led the original phone-hacking investigation, was criticised after he was made a columnist for the Times.

Toxic allegations that the Yard failed to take allegations of endemic phone hacking on the News of the World seriously did for the careers of both the Met's commissioner, Paul Stephenson, and his deputy, John Yates.

Now it is the mirror image of this relationship that is damaging the Sun. The paper's journalists are said to be furious that the arrests have been triggered by information supplied to the Yard by the Management and Standards Committee (MSC), an independent committee set up by the New York-based News Corporation, the parent company of News International. Following the first set of arrests, a News International source suggested it was intent on "draining the swamp", a comment that provoked fury among the company's journalists.

In a statement, the MSC said it was ploughing through a mass of information as it seeks to ensure "that unacceptable news gathering practices by individuals in the past will not be repeated". The committee has given a team of advisers, lawyers and forensic IT staff a broad remit to search for any evidence of wrongdoing. Significantly, the investigation is not confined to the now defunct News of the World. Overseen by Will Lewis, the former Telegraph executive, the committee is working closely with the law firm Linklaters, which is conducting a review of all three remaining News International titles – the Sun, the Times and the Sunday Times.

"The MSC is authorised to conduct internal investigations to fulfil its responsibilities in relation to News International's papers," the committee said in a statement. "It has powers to direct News International staff to co-operate fully with all external and internal investigations, and to preserve, obtain and disclose appropriate documents."

Chaired by Lord Grabiner QC and reporting directly to Joel Klein, executive vice-president and board director of News Corp, the committee's team is examining some 300m emails and what one insider described as "masses of hard copy". "They are looking at more information than one person could read in an entire lifetime," the insider said.

What the committee finds has potentially huge implications, not just for Murdoch's UK newspapers, but the mogul's empire, which stretches across Europe to the US, Latin America and Australia. Legal experts speculate that the bribery allegations could lead to the broadcasting watchdog, Ofcom, reviewing Murdoch's stake in Sky television. Under UK law, owners must prove they are "fit and proper" to own media interests. Any evidence suggesting News International titles were engaged in the corruption of officials could also trigger an investigation by the US authorities into breaches of the Foreign Corrupt Practices Act (FCPA) which prohibits corrupt payments to foreign government officials.

It is this – the threat of the cancer spreading outside the UK and eating away at an empire that includes Fox News and 20th Century Fox film studios, and last year had revenues of $34bn (£21.5bn) – that really worries Murdoch's lieutenants.

"We're more frightened by the [US justice department] than we are of Scotland Yard," a source close to News Corp told Reuters last year. "All Scotland Yard can go after is News International, but the justice department can go after all of News Corporation."

One person close to News Corp suggested fears about an investigation being launched under the FCPA were overblown. But News Corp is clearly concerned about the possibility. Last year it hired Mark Mendelsohn, the deputy chief of the fraud section in the criminal division of the US department of justice, and an expert on the act.

Combing through the potential evidence being presented to the MSC is proving painstaking work. Some of it is in the form of emails that had sat for years in the archives of Harbottle & Lewis, the law firm commissioned by News International following the original phone-hacking allegations that saw the News of the World's royal editor, Clive Goodman, jailed in 2007.

A one-paragraph letter supplied by Harbottle & Lewis to News International, and presented to the parliamentary inquiry into phone hacking, stated that senior editors on the paper were unaware of Goodman's "illegal actions". The letter appeared to corroborate claims made by News International executives that there was no evidence criminal activity on the paper went beyond one rogue reporter.

In response to a question by the Tory MP Philip Davies about whether Goodman was working alone, Colin Myler, the News of the World's then editor, said: "I conducted this inquiry with Daniel Cloke, our director of human resources. Over 2,500 emails were accessed because we were exploring whether or not there was any other evidence to suggest, essentially, what you are hinting at. No evidence was found; that is up to 2,500 emails."

But when the former director of public prosecutions, Lord Macdonald, was hired by a law firm acting on behalf of News Corp to review some of the emails a vastly different picture emerged. Macdonald told parliament that it had taken him "about three minutes, maybe five minutes" to determine that the emails contained evidence of possible criminality. "I can't imagine anyone looking at that file and not seeing crime," said Macdonald, who recommended the file should be handed over to the Yard.

In one email contained in the file it is alleged a senior News International journalist agreed a police contact should receive a "four-figure sum" for leaking a confidential document containing the movements, locations and phone numbers of members of the royal family.

Brooks, Coulson's predecessor on the News of the World, told parliament last year she had no knowledge of payments the Sun might have made to police officers in exchange for information. Yesterday's arrests will place this denial once again under scrutiny. It will also add piquancy to the second stage of Lord Justice Leveson's inquiry that reconvenes at the end of this month and will examine "the relationships between the press and police and the extent to which that has operated in the public interest". Leveson's inquiries are going to be made considerably more difficult by the fact that a significant cast of characters will be reluctant to give evidence for fear of prejudicing any future corruption trials.

Coincidentally, Rupert Murdoch is rumoured to be flying in to London this week. Insiders at News Corp, which last week disclosed the phone-hacking scandal had so far cost it almost $200m, maintain it is a prearranged visit that has nothing to do with the latest allegations dogging Murdoch's newspaper interests. They insist Murdoch has no control over the release of evidence to the Met or how it conducts its investigations.

Some News Corp employees have claimed Murdoch will use the trip to reassure Sun staff their title is safe. Anxious journalists on the bestselling paper will look back to last May when Murdoch flew in to tackle the furore engulfing the News of the World, a cash cow that produced annual profits of more than £10m. The paper was closed within two months. The Sun may have its enemies, but many on Fleet Street will be hoping that history is not about to repeat itself.