London (CNN) -- Three one-time insiders at the defunct News of the World tabloid testified Monday before a British government-backed panel investigating press ethics and behavior.





Former chief reporter Neville Thurlbeck -- who has been arrested in connection with a police probe into phone hacking -- said he would not talk about illegal eavesdropping because of the arrest. He has not been charged.





He appeared at the Leveson Inquiry after the paper's most famous undercover reporter, Mazher Mahmoud, known as the "fake sheikh" for the disguise he often adopted to get stories.





He described doing stories on prostitution and drug dealing, but insisted repeatedly the tabloid "never engaged in entrapment."





He defended his methodology, saying criminals had gone to prison because of his work.





"Exposing criminality gives me great satisfaction," said Mahmoud, whose appearance was not televised in order to protect his identity.





But he said he had never heard about phone hacking by the News of the World until royal reporter Clive Goodman was arrested for it in 2006.





The inquiry was prompted by widespread public outrage this summer at the revelation that the Rupert Murdoch-owned tabloid had hacked into the voice mail of a murdered 13-year-old girl in 2002.





Murdoch's son James, the chief executive of the News Corporation subsidiary that published the tabloid, has repeatedly denied knowing about the scale of illegal eavesdropping at his papers.





James Murdoch ordered the paper shut down over the scandal.





Former News of the World deputy editor Neil Wallis also testified Monday. Like Thurlbeck, he has been arrested and released on bail in a police investigation into phone hacking.





He argued that Britain's downmarket tabloids were the "ones that people out there want to read," and far outsold so-called quality papers such as the Guardian and the Times.





A former News of the World journalist testified last month that the editors of the tabloid knew that their reporters were hacking phones in search of stories.





Paul McMullan named Andy Coulson, who went on to become an adviser to British Prime Minister David Cameron, and Rebekah Brooks, a Murdoch protege, as editors who were aware of the practice.





Coulson resigned as editor of the tabloid in 2007 when Goodman went to prison for hacking the voice mails of Prince William's staff, and he later became Cameron's communications director.





Coulson has always denied knowing about phone hacking, saying he quit the paper because he was ultimately responsible for the actions of his staff.





The Leveson Inquiry has been hearing from high-profile figures since last month, including celebrities such as "Harry Potter" author J.K. Rowling and actor Hugh Grant, both of whom complained about press intrusion into their lives.





Police investigating phone hacking by journalists say that about 5,800 people, including celebrities, crime victims, politicians and members of the royal family, were targets of the practice by journalists in search of stories.





It involves illegally eavesdropping on voice mail by entering a PIN to access messages remotely.
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