Dark rumors 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.

James and Rupert Murdoch. Photograph: Sang Tan/AP

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, is 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 (N.I.), 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 Defense 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.