The passport files of the three leading US presidential candidates have been improperly accessed, the state department has said.
A state department employee looked at the files of both Republican candidate John McCain and Democratic front-runner Barack Obama, the department said.
State department spokesman Sean McCormack said a trainee also accessed Hillary Clinton's file last year.
Two state department contractors have been fired and a third disciplined.
Mr Obama said it was "deeply disturbing" and called for a congressional inquiry into the case.
"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 is functioning," he said.
After first revealing that Secretary of State Condoleezza Rice had apologised to Mr Obama over the security breach, the state department acknowledged Mr McCain's and Mrs Clinton's files had been snooped into as well.
"In the case of Senator McCain, we detected earlier this year... one of the same people who accessed Senator Obama's passport file also accessed Senator McCain's passport file," Mr McCormack said.
That contract employee had been reprimanded but not fired, he said.
The state department has not revealed which company the contractors worked for.
The Obama campaign raised the possibility of political motivation
US Secretary of State Condoleezza Rice has called all three candidates to apologise.
Mr McCormack said the breaches of Mrs Clinton's and Mr McCain's files were discovered on Friday after officials were informed by a reporter on Thursday of the unauthorised access to Mr Obama's record.
He said Mr McCain's passport file had been looked at some time earlier this year.
The Obama breaches occurred on three separate dates - 9 January, 21 February and 14 March.
Mr McCormack said it was not clear what the employees may have seen or what they were looking for.
The state department is investigating the incidents and may call in the Department of Justice if needed.
The state department tracks those who access its passport database. The unauthorised viewings of Mr Obama's file were noticed at a lower level, Mr McCormack said, but senior management was not informed.
A spokesman for Mr Obama suggested that the government could be using private information for "political purposes".
The spokesman, Bill Burton, denounced "an outrageous breach of security and privacy", and called for a full investigation.
Passport files in the US contain personal data such as age and place of birth, as well as a person's foreign travel record.
They also contain an individual's Social Security number, which can be used to obtain credit records and other personal information.
Mr McCain said any breaches of passport privacy called for an apology and a full investigation. He said "corrective action should be taken".
Mrs Clinton's senate office released a statement saying: "Senator Clinton will closely monitor the state department's investigation into this and the other breaches of private passport information."
Her husband Bill Clinton's passport records were breached by a Republican appointee to the state department when he was running for president against George H W Bush in 1992.