News Corp. executive James Murdoch says in a letter to to British parliamentarians investigating the phone-hacking scandal says he could have been more persistent in asking questions about the problem. (David Moir/Reuters)

News Corp. executive James Murdoch acknowledged Wednesday that he could have done more to get to grips with the phone hacking scandal that has rocked Britain and threatened his place as the likely heir to his father's global media empire.

Murdoch's admission came in a seven-page letter written to British parliamentarians investigating the scandal. In it, the 39-year-old repeated his insistence that he didn't know the extent of the illegal behaviour at his now-defunct News of the World tabloid newspaper, saying that the details had been hidden from him by members of his staff.

"It would have been better if I had asked more questions," Murdoch told the House of Commons' media committee. "However, the truth is that incomplete answers and what now appear to be false assurances were given to the questions that I asked."

Murdoch has already appeared twice before lawmakers, who grilled him in detail about what he knew about the phone hacking scandal and alleged attempts to conceal evidence of illegal activity.

Murdoch was the one who signed off on a substantial settlement to one of the first known victims of the practice. The company's former in-house lawyer has said the payoff was aimed at keeping a lid on the scandal, but Murdoch says he had no knowledge of wider wrongdoing and was merely following expert advice.

Critics say Murdoch was either in on the coverup or too incompetent to realize what he was agreeing to, with lawmaker Tom Watson famously accusing Murdoch of being "the first mafia boss in history who doesn't know he's at the head of a criminal enterprise."

In his letter, Murdoch mounted his most detailed defence yet, accusing his former lieutenants of working behind his back, acting without his authorization, and giving inconsistent testimony to Parliament.

In contrast, he said, "My evidence has always been consistent."

It's not certain whether lawmakers will accept that last claim.

Murdoch initially denied knowing anything about a critical piece of evidence which suggested, as far back as 2008, that illegal behaviour went much further than was being publicly acknowledged.

Contradicted by former members of staff, Murdoch later changed his story, saying that while he was told about the damning evidence, its importance wasn't explained to him.

The phone hacking scandal forced the News of the World tabloid to cease publication. Its last issue was on July 9, 2011. (Paul Hackett/Reuters)

News International's attempts to conceal the scope of the scandal fell apart after the Guardian and The New York Times revealed that phone hacking was endemic at the News of the World, an exposé that has led to the paper's closure and the arrests of dozens of journalists and other officials.

The British newspaper arm of Rupert Murdoch's News Corp. has made cash settlements to 58 victims, including celebrities, politicians and the families of crime victims.

James Murdoch himself has resigned from News International, although he retains a senior position in News Corp. and said in his letter that those who saw his resignation as a tacit admission of guilt were wrong.

"I have not misled Parliament," he said. "I did not know about, nor did I try to hide, wrongdoing."

In a separate development, police said that a 51-year-old man was arrested Wednesday on suspicion of intimidating a witness.

Scotland Yard said the man taken into custody had been previously arrested on April 5, 2011. Police did not identify the man, but The

Associated Press had identified a man arrested that day as former News of the World reporter Neville Thurlbeck, who was 50 at the time.

Thurlbeck did not immediately return a text message seeking details about the arrest. His law firm had no immediate comment.

Wednesday's developments follow the arrests of six other suspects, including former News International executive Rebekah Brooks.