Rice said an the Inspector General's office was to investigate the incident.
 
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She added that the unauthorised looks were discovered at a lower level
within the department but that senior management had not been
notified.
 
Sean McCormack, a state department spokesman said it appeared that "imprudent curiosity" was the reason for three separate breaches of Obama's personal information.
 
He said the breaches occurred in January, February and March, and were detected by an internal monitoring system.
 
The state department would not release the names of the contractors involved, or the names of the two companies they worked for.
 
'Political purposes'
 
Bill Burton, a spokesman for Obama's presidential campaign, condemned the Bush administration over the incident.
 
"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," he said. 
 
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"Our government's duty is to protect the private information of the American people, not use it for political purposes.
 
It is not clear what information the employees gained access to. 
 
Personal data such as name, citizenship, age, social security number and place of birth, which is required when a passport application is completed.
 
In another development, Bill Richardson, the governor of New Mexico, endorsed on Friday Obama's campaign, calling him a "once-in-a-lifetime leader" who can restore America's international image.
 
As a Democratic superdelegate, the former presidential hopeful plays a part in the tight race for nominating votes and could bring other superdelegates to Obama's side. He also has been mentioned as a potential running mate for Obama.