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Police so far have arrested seven people for questioning in their investigation of phone hacking and two others in a separate investigation of alleged bribery of police officers. No one has been charged.

Mr. Coulson, Mr. Cameron’s communications director from 2007 until January this year, was arrested Friday.

Ms. Brooks was editor of News of the World in 2002 at the time of the most damaging allegation so far — that the paper hacked into the phone of teenage murder victim Milly Dowler in 2002 and may have impeded a police investigation into the 13-year-old’s disappearance. Ms. Brooks has said she was unaware of any phone hacking at the time.

Rupert Murdoch‘s hope of making BSkyB a wholly owned part of his News Corp. empire collapsed Wednesday in the face of what Mr. Cameron called a “firestorm” that has engulfed media, police and politicians.

Mr. Cameron has appointed a judge for a wide-ranging inquiry into the News of the World scandal and wider issues of media regulation, the relationship between politicians and media, and the possibility that illegal practices are more widespread in the industry.