Hatch, R-Utah, said preliminary interviews suggested that a former Republican member of the committee staff may have also been involved in penetrating the Democratic computers.
"I was shocked to learn that this may have occurred," Hatch said in a statement. "I am mortified that this improper, unethical and simply unacceptable breach of confidential files may have occurred on my watch."
Hatch launched an investigation after Sens. Dick Durbin, D-Ill., and Edward Kennedy, D-Mass., protested what they said was the theft of memos from their servers. The memos, concerning political strategy on blocking confirmation of several of President Bush's judicial nominations, were obtained and reported on by The Wall Street Journal and The Washington Times.
According to the Washington Times, one memo to Durbin singled out Bush nominee Miguel A. Estrada as "especially dangerous" because "he is Latino." Estrada withdrew his nomination after Senate Republicans failed to get a vote on his nomination because of a Democratic filibuster.
In another memo, Kennedy staff suggested another Bush nominee be delayed until the court to which she was nominated finished up an important affirmative action case.
"The thinking is that the current 6th Circuit will sustain the affirmative action program, but if a new judge with conservative views is confirmed before the case is decided, that new judge will be able, under 6th Circuit rules, to review the case and vote on it," the staffers wrote.
The Times said its source was "not a Senate staffer."
Senate Sergeant-at-Arms William Pickle informed Hatch on Monday that the committee's four computer servers had been disconnected and that daily backup tapes had been given to the U.S. Capitol Police for safekeeping. He said an outside expert would conduct a forensic assessment to determine if there had been unauthorized access to files.
Hatch said that, at his direction, two federal prosecutors assigned to the committee had conducted interviews with about 50 people.
He said the interviews revealed that at least one current staff member had improperly accessed at least some of the documents that appeared in the media reports and which have been posted on the Internet. The person has denied leaking the information to the press, he said.
The staff member, who was not identified, was put on administrative leave with pay pending the outcome of Pickle's investigation, Hatch said.
Several leak investigations are under way in Washington. The FBI is investigating whether White House or Pentagon staff members illegally leaked the name of a CIA operative to reporters writing about the operative's husband, former ambassador and Iraq war critic Joseph Wilson.
The Justice Department is also looking into the leak of a classified memo from the Pentagon to the Senate Intelligence Committee. The memo, a reply to the committee's queries about the accuracy of prewar intelligence on Iraq, reports alleged evidence of a link between the Saddam Hussein regime and the al Qaeda terrorist network.
Sen. Jay Rockefeller of West Virginia, the top Democrat on the Senate Intelligence Committee, has suggested that someone may have improperly accessed a staff member's computer to obtain a draft memo outlining a plan for exploiting for political advantage the questions over Iraq's weapons.
Senate Democrats have used filibusters to block votes on six of Mr. Bush's judicial nominees, claiming the nominees' conservative views are too far outside the mainstream.
Republicans have demanded up-or-down votes on the nominees, and have accused the Democrats of violating their constitutional duty to "advise and consent" to the president's judicial picks. The GOP staged a two-day, nonstop debate last week to bring attention to the stifled nominees.
Democrats have allowed votes on around 170 of the president's court nominees. The GOP-controlled Senate blocked 55 Clinton nominees.