WASHINGTON — State Department employees snooped through the passport files of three presidential candidates — Senators Obama, Clinton, and McCain — and the department’s inspector general is investigating.

A State Department spokesman, Sean McCormack, said the violations of Mr. McCain and Mrs. Clinton’s passport files were not discovered until today, after officials were made aware of the unauthorized access of Mr. Obama’s records and a separate search was conducted.

The incidents raise questions as to whether the information was accessed for political purposes and why two contractors involved in the Obama search were dismissed before investigators had a chance to interview them. It recalled an incident in 1992, when a Republican political appointee at the State Department was demoted over a search of presidential candidate Bill Clinton’s passport records. At the time, Clinton was challenging President George H.W. Bush.

Mr. McCormack said one of the individuals who accessed Mr. Obama’s files also reviewed Mr. McCain’s file earlier this year. This contract employee has been reprimanded, but not fired. The individual no longer has access to passport records, 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.

Secretary of State Rice spoke with all three candidates today and expressed her regrets. In the meantime, State Department officials headed to Capitol Hill to brief the candidates’ staffs.

After speaking with Mr. Obama, Ms. Rice told reporters: “I told him that I was sorry, and I told him that I myself would be very disturbed.”

Mr. Obama said Congress should be part of any investigation.

“When you have not just one but a series of attempts to tap into peoples’ personal records, that’s a problem not just for me but for how our government functions,” Mr. Obama told reporters in Portland, Ore. “I expect a full and thorough investigation. It should be done in conjunction with those congressional committees that have oversight so it’s not simply an internal matter.”

The State Department said the Justice Department would be monitoring the probe in case it needs to get involved.

Attorney General Mukasey said the case has not yet been referred to the Justice Department for investigation, and indicated prosecutors likely would wait until the State Department’s inspector general concludes its inquiry. But General Mukasey did not rule out the possibility of the Justice Department taking an independent look at the passport breach.

“Have they asked us to become involved — no,” General Mukasey told reporters during a briefing today. “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, General 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.”

In Mrs. Clinton’s case, an individual last summer accessed her file as part of a training session involving another State Department worker. Mr. McCormack said the one-time violation was immediately recognized and the person was admonished.

Mr. Obama’s records were accessed without permission on three separate occasions — January 9, February 21, and as recently as last week, on March 14.

Mr. McCormack declined to name the companies that employed the contractors, despite demands by a senior House Democrat that such information is in the public interest.

“At this point, we just started an investigation,” he said. “We want to err on the side of caution.”

Mr. McCain, who was in Paris today, said any breach of passport privacy deserves an apology and a full investigation.

“The United States of America values everyone’s privacy and corrective action should be taken,” he said.

It is not clear whether the employees saw anything other than the basic personal data such as name, citizenship, age, Social Security number, and place of birth, which is required when a person fills out a passport application.

Aside from the file, the information could allow critics to dig deeper into the candidates’ private lives. While the file includes date and place of birth, address at time of application, and the countries the person has traveled to, the most important detail would be their Social Security number, which can be used to pull credit reports and other personal information.

The firings and unspecified discipline of the third employee already had occurred when senior State Department officials learned of the break-ins to the files. Ms. Rice learned about it yesterday, after a reporter inquired about Mr. Obama’s case.

The violations were detected by internal State Department computer checks because certain records, including those of high-profile people, are “flagged” with a computer tag that tips off supervisors when someone tries to view the records without a proper reason.

The Washington Times first reported the incident involving Obama.

A former independent counsel, Joseph diGenova, said the firings of the contract employees will make the investigation more difficult because the inspector general can’t compel them to talk.

“My guess is if he tries to talk to them now, in all likelihood they will take the Fifth,” Mr. diGenova said, referring to the Fifth Amendment’s protection against self-incrimination.

The State Department’s top management officer, Undersecretary Patrick Kennedy, briefed members of the Clinton, Obama, and McCain staffs in a Senate Foreign Relations Committee room midday today.

“Mistakes and errors happen from time to time. … We caught these and we’ve got to work and correct that process,” Mr. Kennedy said after the more than 90-minute session.

Mr. Kennedy had said yesterday that the incident was not handled properly.

“I will fully acknowledge this information should have been passed up the line,” Mr. Kennedy told reporters in a conference call. “It was dealt with at the office level.”