WASHINGTON — Maybe it was just curiosity.

But whatever led State Department workers to pry into the passport files of Hillary Rodham Clinton, Barack Obama and John McCain, it wasn't an accident.

Each time agency workers go into someone's electronic personal files, the system reminds them that such information is restricted under privacy laws and may be reviewed only on a need-to-know basis. It also reminds them that penalties will be imposed if the information is being improperly checked.

The warning wasn't enough for at least four department workers who were caught prying into the presidential candidates' files, an embarrassing episode that led Secretary of State Condoleezza Rice to telephone the candidates with her personal apology yesterday and to promise a full investigation.

The snooping incidents raised questions as to whether there was political motivation and why two contractors involved were fired before investigators had a chance to interview them. The State Department's inspector general was probing, with the Justice Department monitoring the effort, but Obama said that was not enough. He urged congressional involvement "so it's not simply an internal matter."

McCain said there should be an investigation of the new snooping as well as an apology.

In all, at least four workers were involved in the snooping.

The State Department confirmed Thursday night that Obama's files had been compromised on three occasions — Jan. 9, Feb. 21 and March 14. By the time senior officials were made aware, two contract employees had been fired and a third disciplined, agency officials said.

The companies that provided the contractors were The Analysis Corp. and Stanley Inc. Stanley, based in Arlington, Va., this week won a five-year, $570 million government contract extension to support passport services.

According to agency officials, a Stanley employee improperly accessed Obama's records on Jan. 9 and was fired within days. The second contractor, employed by The Analysis Corp., pried into similar records on Feb. 21 but was not terminated. The third incident involved another Stanley employee, who was swiftly fired.

In a statement, Stanley said it regretted the "rare occurrence" of unauthorized access of any individual's private information.

When it fired the two subcontractors involved in accessing the Obama files, terminating them the day the incident was discovered, Stanley was not aware of others performing unauthorized searches of McCain or Clinton's files, the company said.

The Analysis Corp. issued a statement yesterday saying it had been notified earlier in the day that one of its contractors had acted improperly. The company said it had decided to honor the department's request to delay firing its consultant to give investigators time to conduct its investigation.

State Department spokesman Sean McCormack said yesterday that a separate search conducted after the Obama revelation showed that workers also had snooped on McCain and Clinton.

The worker who had been reprimanded in the Obama incident also had reviewed McCain's records earlier this year, McCormack said. While the employee has not been fired, that person no longer has access to passport files, he said.

In Clinton's case, someone accessed her file last summer. McCormack said the violation was immediately recognized and the person was admonished. That person was not involved in the later incidents, meaning four people were involved in all.