Britain's Supreme Court took a step toward exposing the names at the heart of Britain's phone hacking scandal Wednesday, ruling that a private investigator convicted of eavesdropping for a Rupert Murdoch-owned tabloid must reveal who ordered him to do it.

Meanwhile, a police investigation into press wrongdoing triggered by the hacking revelations expanded beyond Murdoch's media empire with the arrest of a former reporter from the rival Mirror group.

Private eye Glenn Mulcaire was jailed briefly in 2007 for hacking the voicemail messages of royal aides on behalf of the now-defunct News of the World.

Hacking victims suing Rupert Murdoch's News International want Mulcaire to provide evidence for their cases by identifying the editor who told him to hack the phones. The case before the court relates to a lawsuit by Nicola Phillips, an assistant to PR guru Max Clifford, who claims her phone was hacked.