Scotland Yard is wading through 300 million News International emails as part of its widening probe into phone-hacking at Rupert Murdoch's 'News of the World' tabloid.
The new Metropolitan Police Commissioner Bernard Hogan-Howe revealed the figure to journalists when asked about the scope of the inquiry.
He said 600 phone-hacking victims had been interviewed by police out of a total of 5,700 potential victims of phone-hacking at the now-defunct 'News of the World'.
His comments came as News International's chief executive James Murdoch rejected an accusation in a Commons select committee by Labour MP Tom Watson that the company had behaved like the mafia over the hacking scandal and operated a code of silence.
Murdoch's media empire is under pressure for using unethical and illegal practices such as phone-hacking, computer hacking, 'pinging' (tracking location of mobile phones), 'blagging' (masquerading as someone else to secure data by phone) and covert surveillance.
The inquiry had so far cost the agency three million pounds in salaries alone, the BBC quoted Bernard as saying.
He said Scotland Yard had also spoken to a further 1,200 people who thought their phones had been hacked but had since been reassured that it was not the case.
"Some were people in public life who had things happened to them they couldn't explain and they thought 'maybe I was phone-hacked' and they contacted us," Bernard said.
He said that to speed up the task of talking to victims, police were now asking them to come to Scotland Yard rather than travelling to them.
Around 100 detectives are working on the three inquiries:
Operation Weeting - into the phone-hacking itself; Operation Elveden - into allegations of corrupt payments by journalists to police officers and Operation Tuleta - into the possibility that emails may have been intercepted.
He conceded that 100 people was a lot to put on one case and that most of London's murder cases had far fewer detectives working on them.
But he stressed that it was important to investigation the allegations properly to reassure the public.
At Thursday's hearing, James Murdoch, the son of News Corp chief Rupert Murdoch, also repeated his apologies for the scandal which led to the closure of the newspaper in July after 168 years.
"The whole company is humbled by this... We are all humbled by it and trying to improve the business, improve the structures and leadership," James Murdoch said.