London's Metropolitan Police would not comment, saying the force did not identify suspects who have not been charged.
The force also declined to comment on reports that Murdoch's News U.K. is under criminal investigation.
The investigation initially focused on Murdoch's papers, but has spread to take in other companies. Several former Trinity Mirror staff have been arrested, including a former editor and deputy editor of the Sunday Mirror. Last week former Sunday Mirror and News of the World journalist Dan Evans was charged with phone hacking.
CNN interviewer Piers Morgan, a former editor of the Sunday title's sister paper, the Daily Mirror, told an inquiry into media ethics that he was not aware of any such wrongdoing while he led the newspaper between 1995 and 2004.
The head of the inquiry, Justice Brian Leveson, called Morgan's claim "utterly unpersuasive," and said hacking may well have occurred at the Mirror in the late 1990s.
Trinity Mirror shares fell almost 4 percent on the London Stock Exchange after the announcement, before recovering to stand at 1.25 pounds ($1.98), about 2.5 percent down.