The US Department of State has launched an investigation into repeated breaches of passport files compiled on the three main presidential candidates.
State department contractors viewed the files of Republican John McCain and Democrat Barack Obama, while a trainee accessed Hillary Clinton's file.
Secretary of State Condoleezza Rice has apologised to all three candidates.
US passport files include data such as age and place of birth, foreign travel records, and a Social Security number.
That number can be used to obtain credit records and other personal information.
Two state department contractors have been fired and a third disciplined.
The case has raised suspicions that someone was digging for information that could be damaging to the candidates.
However, with breaches concerning all three contenders, the likelihood of political motivation is somewhat diminished, the BBC's Kim Ghattas reports from Washington.
On Friday the state department said the employees who had been disciplined worked for two contractors, Analysis Corporation and Stanley.
Stanley is an IT firm that recently won a 5-year contract to issue American passports worth more than $500m (£252m).
Mr McCain's file was looked at earlier this year
There are now concerns over why the state department fired two contractors before having time to question them, correspondents say.
The Department of Justice is to monitor the state department's probe, while the US House of Representatives Foreign Affairs Committee has also said it will launch an investigation.
Mr Obama called the case "deeply disturbing" and asked for a congressional enquiry.
"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.
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 said it would closely monitor the state department's probe.
The passport records of her husband, Bill, were breached by a Republican appointee to the state department when he was running for president against George H W Bush in 1992.
After first revealing that Ms Rice had apologised to Mr Obama over the security breach, the state department acknowledged Mr McCain's and Mrs Clinton's files had been looked at as well.
It said that one contract employee had viewed the files of Mr Obama and Mr McCain, and had been reprimanded but not fired.
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.
The state department tracks the viewing of its passport database.