In Clinton's case, an individual last summer accessed her file as part of a training session involving another State Department worker. McCormack said the one-time violation was immediately recognized and the person was admonished.
Obama's records were accessed without permission on three separate occasions - Jan. 9, Feb. 21 and as recently as last week, on March 14.
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."
McCain, who was in Paris on Friday, 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. Rice learned about it Thursday, after a reporter inquired about 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.
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," 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 Friday.
"Mistakes and errors happen from time to time. ... We caught these and we've got to work and correct that process," Kennedy said after the more than 90-minute session.
Kennedy had said Thursday that the incident was not handled properly.
"I will fully acknowledge this information should have been passed up the line," Kennedy told reporters in a conference call. "It was dealt with at the office level."
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