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London (CNN) -- A British police investigation into illegal phone hacking by journalists has expanded beyond Rupert Murdoch's disgraced News of the World tabloid to many other newspapers, the British Information Commissioner's Office told CNN Thursday.

The Metropolitan Police asked the data-protection agency to hand over files from a 2006 investigation into the work of a private investigator who sold illegally obtained information to a wide range of newspapers, including the Daily Mail, Daily Mirror and News of the World.

The scandal over phone-hacking by News of the World forced the paper to close, prompted two top police officers to resign, and has put pressure on Prime Minister David Cameron, who hired a former editor of the paper to be his spokesman.

Police added 15 more officers to the investigation, boosting the existing team to 60, the officer in charge of the case said Wednesday.

The increase comes in response to "a surge of enquiries and requests for assistance from the public and solicitors," Deputy Assistant Commissioner Sue Akers said.

Police were already plowing through 11,000 pages of documents seized from a private investigator before they requested the additional files from the Information Commissioner.

The request for files from the Information Commissioner investigation came three months ago, the commissioner's office said Thursday.

Its 2006 report, "What Price Privacy Now?," names 31 newspapers and magazines that were clients of a private investigator who illegally obtained private information about people, usually by calling and pretending to be someone he was not.

The Daily and Sunday Mail newspapers, and the Daily Mirror and associated Sunday People, hold the top four spots. News of the World comes fifth.

The Mail and Mirror did not immediately respond to CNN requests for comment.

Piers Morgan, who now presents a CNN interview program, was editor of the Daily Mirror during part of the period covered by the report.

He has vehemently denied ever engaging in phone hacking, either when he was at the Mirror or before that, when he was at News of the World.

"In my time at the Mirror and the News of the World, I have never hacked a phone, told anybody to hack a phone or published any story based on the hacking of a phone," Morgan said on CNN Tuesday.

News of the World closed 10 days ago -- after 168 years -- in the face of public outrage over the accusation that people working for it had hacked the phone of Milly Dowler, a missing 13-year-old who was later found dead.

The Information Commissioner report does not deal with phone hacking, but with "blagging," as British journalists call illegal impersonation to get private information.

The prime minister is caught up in the scandal because he hired a former News of the World editor to be his spokesman after the journalist, Andy Coulson, resigned over a separate phone-hacking case. Coulson has always denied wrongdoing but quit when a journalist working for him pleaded guilty to hacking.

Cameron said Wednesday that in hindsight, he should not have hired Coulson. Coulson, who resigned his government post in January, has since been arrested.

News of the World journalists are accused of hacking into the voice mail of potentially thousands of people and of bribing police. The scandal has rattled the foundations of the British press, police and political establishments.

It also forced media baron Rupert Murdoch, who owned the paper, to testify before Parliament this week alongside his son James, a top News Corp. executive.

They denied knowing about illegal activities by their employees and vowed to clean house.

Asked by one lawmaker, "Do you accept that ultimately you are responsible for this whole fiasco?" Rupert Murdoch responded: "No."

After declaring it was "the most humble day of my life," the elder Murdoch let James Murdoch do most of the talking. When called upon, Rupert Murdoch said he knew little of the day-to-day details of his holdings and that he might hear more from a News of the World editor about extra soccer coverage than a payout to a phone-hacking victim.

Cameron has announced an inquiry into press practices, led by a judge and assisted by civil rights campaigners and former top journalists and police officers.

Murdoch's News Corp. encompasses Fox News, The Wall Street Journal, the New York Post, and Harper Collins publishers in the United States. News International -- a British subsidiary of News Corp. -- owns the Sun, The Times and The Sunday Times in Britain.