"We believe this was out of imprudent curiosity, so we are taking steps to reassure ourselves that that is, in fact, the case," McCormack said.
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," Burton said. "Our government's duty is to protect the private information of the American people, not use it for political purposes."
"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.
McCormack said it was not immediately clear what the contract employees may have seen in the records or what they were looking for. He said he did not know the names of the companies they worked for.
The department has informed Obama's Senate office of the breach, and a personal briefing for the Illinois senator's staff was scheduled for Friday, McCormack said.
Similar breaches involving public officials have happened in the past.
During the 1992 presidential campaign, officials in the administration of President George H.W. Bush searched the State Department files of then-Democratic nominee Bill Clinton. An inspector general's report called the search improper and said it was aimed at finding material that would be damaging to Clinton's campaign.
At the time of the searches, which took place Sept. 30 and Oct. 1, 1992, Republicans were criticizing Clinton for anti-Vietnam War activities when he was a student at Oxford University in England in 1969-70. Officials looked into the passport files after rumors sparked freedom of information requests from the news media and a Republican congressman, but later discovered that privacy laws would bar their disclosure, the report said.
After a three-year investigation costing $2.2 million, independent counsel Joseph diGenova concluded in a separate report in 1995 that some of the actions investigated had been "stupid, dumb and partisan" but not criminal.
