By Mark Hosenball

LONDON Feb 27 (Reuters) - A massive email cache that a corporate clean-up team has assembled as part of its effort to cooperate with British police contains message traffic generated by journalists at all four British newspapers once published by Rupert Murdoch, sources familiar with the unit's work said.

The clean-up team, known as the Management and Standards Committee (MSC) of Murdoch's US-based News Corp, is investigating reporting practices across all Murdoch's current and former UK properties, said one of the sources familiar with the company's internal investigations. These include the now-defunct News of the World as well as The Sun, the Times and the Sunday Times, all of which still publish.

An "investigation across all News International titles remains ongoing," the source said.

From fragments of data which company officials allegedly tried to delete, the MSC and outside consultants managed to assemble a data base containing an estimated 300 million emails covering roughly the last decade, the source said.

A team of police investigators has set up shop in an office suite close to a separate suite in Murdoch's newspaper publishing campus occupied by MSC members and a battery of outside lawyers.

However, the source familiar with its work says that the MSC has worked out a set of procedures with the police which the company team believes constitutes an effective mechanism for protecting journalistic sources.

According to the source, the procedure works this way: the police team embedded at Murdoch's complex in London's Wapping district provide the MSC with "search terms" which MSC representatives and their legal advisors then use to tap into the 300 million email database.

If they find message traffic relevant to the search terms supplied by police, the source said, before being handed over to police that traffic is reviewed by MSC officials and lawyers from the London firm Linklaters to see if it contains information which should be redacted. This could include sensitive information covered by legal privilege or which would identify confidential sources.

Only after a careful legal review, and the redaction of sensitive material based on the advice of lawyers, is the material from the data bank turned over to police investigators, the source familiar with the procedure said.

Police have "no live access" to the underlying data pool, the source said. However, under the procedure they can certainly ask follow up questions after receiving censored data to seek additional information or searches.

In public testimony on Monday before an inquiry into British reporting practices headed by High Court judge Brian Leveson, Sue Akers, the Deputy Assistant Commissioner of London police in charge of three parallel inquiries into potentially illegal reporting tactics said that police had sought advice from prosecutors on how to investigate journalists and newspaper offices.

Akers said that in unspecified instances where "there is an evidential base to request information, the MSC have provided it in unredacted format to enable police to identify the public official concerned."

However, she added, the MSC is providing police with information in redacted form in connection with their more general investigation of cash payments.

She said that the sources' names would remain redacted "until police are able to produce evidence that can justify identifying the source."

She said that initially, a police team assigned to Operation Elveden, which is specifically focused on questionable payments to police and other public officials, based on material supplied to it by Murdoch's News International focused on journalists from the News of the World.

Murdoch closed that newspaper last summer amid uproar over the alleged involvement of the paper's journalists in widespread voice-mail hacking.

More recently, Akers said, police expanded Operation Elveden to include journalists on The Sun, whose first Sunday edition was personally launched by Murdoch last weekend. As a result of information provided to police by the MSC, Operation Elveden inquiries led to the arrest of 10 Sun journalists since last November, including some of the paper's longest-serving and most senior employees.

None of those arrested have been charged with violating any law.

In her testimony, Akers stressed that her investigation was not interested in petty dealings between journalists and sources, such as the buying of drinks or meals.

She alleged that Operation Elveden had found evidence of a "network of corrupted officials" in the police, military, UK health service, government and prison service, and that there had been a "culture at The Sun of illegal payments" as well as systems in place to hide the identities of officials receiving money.

She said that in one case, emails revealed that one unnamed person received payments which totalled more than 80,000 British pounds. One of the journalists who has been arrested, Akers said, over several years received more than 150,000 pounds in cash to pay his sources, "a number of whom were public officials."

(Reporting By Mark Hosenball)

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