WASHINGTON -- State Department workers improperly snooped in the passport files of all three major presidential candidates, officials said Friday, a disclosure that prompted Secretary of State Condoleezza Rice to take the unusual step of delivering a round of personal apologies.

An investigation begun this week revealed that since last summer, a State Department staff trainee and three contract workers in the department's passport office had poked through passport application files containing the Social Security numbers and other personal information of the presidential hopefuls.

Officials said they think the workers were motivated by nothing more than "imprudent curiosity," but they have not ruled out more serious motives and have asked the department's inspector general to investigate.

Rice told reporters Friday morning that she had called Sen. Barack Obama (D-Ill.) to apologize.

"I myself would be very disturbed if I learned that somebody had looked into my passport file," Rice said she told Obama. "None of us wants a circumstance where any American's passport files are looked at in an unauthorized way."

Rice later called Sens. Hillary Rodham Clinton (D-N.Y.) and John McCain (R-Ariz.) to express similar sentiments and to promise that the State Department would get to the bottom of the issue, officials said.

The incidents came to light Thursday evening with the disclosure that the three contract workers had rifled Obama's file between Jan. 9 and March 14. State Department officials began conducting additional checks and by Friday morning had discovered that supervisors in the passport office knew of incidents involving all three candidates.

The disclosures prompted concern from the candidates and alarm from privacy advocates and critics of the Bush administration. Theories that the incidents were driven by political motives permeated the blogosphere.

In 1992 the passport file of then-presidential candidate Bill Clinton was ransacked. A three-year, $2.2-million investigation by an independent counsel concluded that no laws had been broken, but that the officials involved were guilty of actions deemed "stupid, dumb and indeed partisan." A Republican political appointee at the State Department was demoted.

McCain, traveling Friday in Paris, said that any victim of privacy violations "deserves an apology and a full investigation."

Obama said he wanted a congressional probe into the incidents, while Hillary Clinton's campaign said in a statement that she intended to "closely monitor" the State Department's investigation.

Two of the contract workers have been fired. The third has been disciplined, and the trainee has been "admonished."

The three contract workers were among 2,600 contract employees that the State Department uses to help an overburdened operation that last year turned out 18 million passports.

Officials declined to identify the firms employing the disciplined workers.

However, a Virginia firm, Stanley Inc., which holds several large government contracts, confirmed Friday that two employees of a subcontractor were involved in accessing Obama's passport records and were fired the day the unauthorized search took place.

Stanley officials said they didn't know whether any of their employees improperly snooped in files for McCain or Clinton.

"While this is a rare occurrence, we regret the unauthorized access of any individual's private information," the company said in a statement.

Stanley was awarded a new, $570-million passport contract this week; it also runs government facilities in Vermont, Arizona, Arkansas and California.

Its California contract is for work at a Laguna Niguel office that processes immigration and citizenship applications.

The State Department will try to determine whether the workers broke any laws and whether the supervisors broke any rules by failing to inform superiors of the incidents.