Wagner's agreement last Friday to plead guilty to federal charges in the HP spying case gives prosecutors the power to charge others in the case, a legal expert said.

Wagner, a 29-year-old US private investigator, pleaded guilty to conspiracy and aggravated identity theft charges in a federal court in California. He has agreed to cooperate with prosecutors investigating possible co-conspirators in exchange for possible leniency at sentencing.

"The first thing that a prosecuting attorney needs in a case like this is an inside witness," said Franklin Zimring, a criminal justice law professor in the Earl Warren Legal Institute at the US University of California. "Without that, it's a question of inferences from circumstantial evidence, and no fun at all for the prosecutor."

The charges stem from an investigation HP conducted in 2005 and 2006 to identify who on the HP board was leaking news of board deliberations to the media.

According to information presented in court Friday by Assistant US Attorney Mark Krotoski, HP hired a Boston private investigative firm, Security Outsourcing Solutions, which in turn hired Action Research Group. Action Research, in turn, hired Wagner, as an independent contractor.

The targets of the investigation included reporters who cover HP, company directors and some family members of both.

Wagner was given social security numbers and other personal information about the targets and used it to pose as those people to get calling records from phone companies, a practice called "pretexting," Krotoski said.

Wagner still faces state felony charges in California along with former HP board chairman Patricia Dunn, former HP legal counsel Kevin Hunsaker, Ronald R. DeLia, of Security Outsourcing Solutions and Matthew DePante, manager of Action Research Group. Wagner is the only person prosecuted in federal court in connection with the case and the first to be convicted.

Attorneys for Dunn and Hunsaker, who were forced out of their HP posts because of the scandal, could not be reached for comment on Wagner's plea.

Wagner only had direct contact with DePante but was assured that lawyers had reviewed the practice of pretexting and said it was legal, said Wagner's attorney, Stephen Nataril. He told reporters after the hearing that his client was "the classic dupe".

He also said that private investigators like those at Action Research Group regularly engage in suspect practices to get information.

The case ultimately may come down to which investigative practices are criminal and which are merely legal "artifice," Zimring said.

"The question of the line between misrepresentations, which are tolerated, and prosecutable fraud is a very difficult border to patrol," said Zimring. "What's the difference between fraud and selling used cars is a philosophical question."