The snooping incidents raised questions as to a political motivation and why two contractors involved were fired before investigators had a chance to interview them. The State Department's inspector general was probing, with the Justice Department monitoring the effort, but Senator Obama said that was not enough.

He urged the involvement of Congress 'so it's not simply an internal matter.' The digging into supposedly secure government records on politicians recalled a 1992 case in which a Republican political appointee at the State Department was demoted for searching Democrat Bill Clinton's passport records when Mr Clinton was running against President George H.W. Bush.

Mr McCain, the Republican nominee-in-waiting, said there should be a full investigation of the new snooping as well as an apology.

Democrat Senator Obama said it should include Congress, not just Bush administration investigators.

'When you have not just one but a series of attempts to tap into people's personal records, that's a problem not just for me but for how our government functions,' Senator Obama told reporters in Portland, Oregon, where he was campaigning. 'I expect a full and thorough investigation. It should be done be in conjunction with those congressional committees that have oversight function so it's not simply an internal matter.'

Dr Rice was apologetic in public as well as in her private phone calls to the candidates.

'None of us wants to have a circumstance in which any American's passport file is looked at in an unauthorised way,' she said after speaking with Senator Obama.

'I told him that I was sorry, and I told him that I, myself, would be very disturbed if I learned that somebody had looked into my passport file,' she added. 'And therefore, I will stay on top of it and get to the bottom of it.'

The State Department confirmed on Thursday night that Senator Obama's files had been compromised on three occasions: Jan 9, Feb 21 and as recently as last week, March 14. By the time senior officials were made aware, two contract employees had been fired and a third disciplined, agency officials said. The fact that the two have been fired could make it more difficult for the State Department to force them to answer questions.

Stanley
The department would not name the company that employed those workers, but The Associated Press learned it was Stanley, a company based in Virginia's Washington suburbs that won this week a five-year, US$570-million (S$790.6-million) government contract to support passport services.

Department spokesman Sean McCormack said on Friday that a separate search conducted after the first revelation showed that workers also had snooped on Mr McCain and Mrs Clinton.

The individual who had been reprimanded in the Obama incident had also reviewed Mr McCain's records early this year, Mr McCormack said.

While the employee has not been fired, that person no longer has access to passport files, he said.

'I can assure you that person's going to be at the top of the list of the inspector general when they talk to people, and we are currently reviewing our (disciplinary) options with respect to that person,' Mr McCormack said.

In Mrs Clinton's case, someone accessed her file last summer as part of a training session involving another State Department worker.

Mr McCormack said the violation was immediately recognised and the person was admonished.

The department's internal computer system 'flags' certain records, including those of high-profile people, to tip off supervisors when someone tries to view the records without an appropriate reason.

'Imprudent curiosity'
Mr McCormack said information on the incidents points to workers' 'imprudent curiosity' more than to something more sinister.

Still, he said, 'we are not dismissive of any other possibility, and that's the reason why we have an investigation under way,' he said.

Likewise, Patrick Kennedy, the top management official at the State Department, briefed the candidates' staffs on Capitol Hill, then said to reporters, 'The State Department has very, very rigorous rules about controls and access for privacy material. We review them regularly and we have a large organisation with a lot of people in it. Mistakes and errors happen from time to time. ... We caught these and we've got to work and correct that process.'

Attorney General Michael Mukasey said the case has not yet been referred to the Justice Department for investigation and indicated prosecutors were likely to wait until the State Department's inspector general ends that inquiry. But Mr Mukasey did not rule out the possibility of the Justice Department taking an independent look.

'Have they asked us to become involved - no,' Mr Mukasey said.

'When, as, and if we have a basis for an investigation, including a reference - that is, one basis would be a reference - we could conduct one.'

Asked what another basis could be, Mr Mukasey said: 'I don't want to speculate, but if somebody walked in here with a box full of evidence, they wouldn't be turned away.' The Washington Times first reported the incident involving Senator Obama. -- AP