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The breaches occurred on Jan. 9, Feb. 21 and March 14, and were detected by internal State Department computer checks, McCormack said. The department's top management officer, Undersecretary Patrick Kennedy, said 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 State Department would not release the names of those who were fired and disciplined, or the names of the two companies they worked for. The department's inspector general is investigating.
"We believe this was out of imprudent curiosity, but we are taking steps to reassure ourselves that that is, in fact, the case," McCormack said.
The firings and unspecified discipline of the third employee already had occurred when senior State Department officials learned of the breaches. Kennedy called that a failing.
"I will fully acknowledge this information should have been passed up the line," Kennedy told reporters in a conference call Thursday night. "It was dealt with at the office level."
Bill Burton, a spokesman for Obama's presidential campaign, called for a complete investigation.
"This is an outrageous breach of security and privacy, even from an administration that has shown little regard for either over the last eight years. Our government's duty is to protect the private information of the American people, not use it for political purposes," Burton said.
"This is a serious matter that merits a complete investigation, and we demand to know who looked at Senator Obama's passport file, for what purpose and why it took so long for them to reveal this security breach," he said.
The department informed Obama's Senate office of the breach on Thursday. Kennedy said that at the office's request, he will provide a personal briefing for the senator's staff on Friday. No one from the State Department spoke to Obama personally on Thursday, the officials said.
Obama was born in Hawaii and lived in Indonesia for several years as a child before returning to the states. As a member of the Senate Foreign Relations Committee, he has traveled to the Middle East, the former Soviet states with Sen. Richard Lugar, R-Ind., and Africa, where in 2006 he and his wife, Michelle, publicly took HIV tests in Kenya to encourage people there to do the same.
Obama's father was born in Kenya, and the senator still has relatives there.
The disclosure of inappropriate passport inquiries 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. The State Department's inspector general said the official had helped arrange the search in an attempt to find politically damaging information about Clinton, who had been rumored to have considered renouncing his citizenship to avoid the Vietnam war draft.
The State Department said the official, Steven Berry, had shown "serious lapses in judgment."
Doug Hattaway, a spokesman for Sen. Hillary Rodham Clinton, the former first lady who is challenging Obama for the Democratic presidential nomination, said of the breach: "It's outrageous and the Bush administration has to get to the bottom of it."
In the current case, Kennedy and McCormack said it was too soon to say whether a crime was committed. The searches may violate the federal Privacy Act, and Kennedy said he is consulting State Department lawyers.
The State Department inspector general's power is limited, because two of the employees are no longer working for the department. McCormack said it was premature to consider whether the FBI or Justice Department should be involved.
McCormack said Secretary of State Condoleezza Rice was informed of the breaches on Thursday.
The State Department conducts background checks of its contract employees who perform passport applications work, but does not ask about political affiliations, Kennedy said.