Scotland Yard has boosted the police team investigating Britain's phone hacking scandal from 45 to 60 officers.
Metropolitan Police Deputy Assistant Commissioner Sue Akers says the move came after a "significant increase in the workload" over the past fortnight, the BBC reports.
The investigation may also be spreading beyond News International's News of the World newspaper.
Police have asked for files of an earlier inquiry into the use of private investigators, the BBC has learned.
According to BBC Radio 4's The Report, the files from Operation Motorman, which was run by the Information Commissioner's Office in 2003, were requested three months ago.
They contain 4,000 requests from 300 journalists and 31 publications for confidential information from a private investigator, which in many cases had been obtained illegally.
The investigation found the Daily Mail had made the most requests, followed by the Sunday People and the Daily Mirror.
The Daily Mail told the BBC the information obtained may have been for reasons of public interest, while Trinity Mirror Group said its journalists worked within the law and the Press Complaints Commission code of conduct.
