Illegal snooping into the passport files of US political candidates may have been more common than reported, state department officials have admitted.

Condoleezza Rice, the secretary of state, has apologised to the three presidential candidates.

The motive for the intrusions by government contractors with access to passport files is unknown. An initial report that personnel had accessed the files of Senator Barack Obama was followed by revelations that Senator Hillary Clinton and Senator John McCain had also been subject to breaches of privacy laws.

Officials said Clinton’s file had been opened by a trainee learning how to use the system. The intrusion was spotted immediately and the trainee admonished.

There was no explanation for the frequent attempts to examine Obama’s personal information, which is likely to have included his birth details, social security number, and personal home and e-mail addresses.

Investigators have established that one of the contractors who accessed Obama’s files also looked at McCain’s details. “There may be more than three [breaches],” a department spokesman said.

The state department has dismissed two employees and reprimanded a third. One was said to have been employed by a company headed by John Brennan, a former CIA agent who is an adviser to Obama’s campaign.

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