HP security official warned others that probe methods were "unethical at the least and probably illegal."
In a Feb. 7 e-mail, HP security official Vincent Nye told the company's head of security and a lawyer supervising a probe into unauthorized leaks that he had "serious reservations" about what the company was doing.
Nye said that his understanding of the methods HP was using to obtain telephone records using false pretenses "leaves me with the opinion that it is very unethical at the least and probably illegal," he said in the e-mail, which was turned over to the House Energy and Commerce Committee and seen by CNET News.com.
"If it is not illegal, then it is leaving HP in a position of (sic) that could damage our reputation or worse," Nye said in the e-mail. "I am requesting that we cease this phone number gathering method immediately and discount any of its information."
The e-mail appears to have been sent to former HP security head Anthony Gentilucci and Kevin Hunsaker, a former HP senior counsel. Both employees left HP as of Tuesday, the company said.
HP declined to comment Wednesday on Nye's e-mail warning.
The disclosure of Nye's warning comes on the eve of a congressional committee hearing into HP's internal investigation of the leaks. The House Energy and Commerce Committee has called on a number of HP executives to appear, including Hunsaker, HP CEO Mark Hurd and former Chairman Patricia Dunn.
Federal and state authorities have also launched criminal probes into HP's use of pretexting, or obtaining telephone records through false pretenses. HP has said its investigators targeted more than a dozen people, including current and former board members, nine journalists, two employees and an unspecified number of others.
Nye is at least the second HP employee to raise a red flag about the technique.
On Jan. 28, Fred Adler, a member of HP's IT security team, was asked if investigators might be able to obtain text messages for former board member Tom Perkins. Adler responded the same day that the company could not legally obtain the records "unless we either pay the bill or get consent." Adler's e-mail was reported last week by The Wall Street Journal.
Meanwhile, also on Wednesday, the House committee released the written testimony of Dunn and Hurd. In her testimony, Dunn said others at the company were involved in directing the leak probe and said that she believed the company was obtaining phone records "in a legal and appropriate manner."
In his written testimony, Hurd apologized, saying that the leak probe evolved into a "rogue investigation."
HP and Dunn have both said that the investigation took part in two phases, with the first stretching from early 2005 through that summer. Dunn has said the second phase resumed in late January 2006 after a Jan. 23 CNET News.com story that included details from a board meeting earlier that month near Palm Springs, Calif.
However, HP disclosed at a press conference on Friday that the company's outside investigators used physical surveillance at a January board meeting. According to sources, the only board meeting that month was the event in Palm Springs. HP declined to comment on why that meeting was watched if its leak investigation was not active.
CNET News.com's Anne Broache and Greg Sandoval contributed to this report.