The companies that provided the contractors were The Analysis Corp. and Stanley Inc. Stanley is a Virginia-based company that earlier this week won a five-year, $570-million government contract extension to support passport services.
According to agency officials, the first 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.
The Analysis Corporation, says it didn't fire its employee at the department's request. The company says the delay was meant to give authorities time to investigate.
The State Department says that person no longer has access to passport files.
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."
The digging into supposedly secure government records on politicians recalled a 1992 case in which a Republican political appointee at the State Department was demoted for searching Bill Clinton's passport records when Clinton was running against President George H.W. Bush.
David Laufman, one of the investigators then, told CBS News correspondent Jim Axelrod that this case sounds different so far.
"It may boil down to simple human folly and voyeurism in the wee hours of the morning," Laufman said, "individuals surfing a database to look for information about famous people."
McCain, the Republican nominee-in-waiting, said there should be "a full investigation" of the new snooping as well as an apology.
Democrat Obama said that better include Congress, not just Bush administration investigators.
"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 functions," Obama told reporters in Portland, Ore., where he was campaigning. "I expect a full and thorough investigation. It should be done be in conjunction with those congressional committees that have oversight function so it's not simply an internal matter."
Rice was apologetic in public as well as in her private phone calls to the candidates.
"None of us wants to have a circumstance in which any American's passport file is looked at in an unauthorized way," she said after speaking with Obama.
"I told him that I was sorry, and I told him that I, myself, would be very disturbed if I learned that somebody had looked into my passport file," she added. "And therefore, I will stay on top of it and get to the bottom of it."
The State Department confirmed Thursday night that Obama's files had been compromised on three separate occasions - Jan. 9, Feb. 21 and as recently as last week, on March 14. By the time senior officials were made aware, two contract employees had been fired and a third disciplined, agency officials said. The fact that the two have been fired could make it more difficult for the State Department to force them to answer questions.
Department spokesman Sean McCormack said Friday that a separate search conducted after the first revelation showed that workers also had snooped on McCain and Clinton.
The individual who had been reprimanded in the Obama incident had also reviewed McCain's records earlier this year, McCormack said.
"I can assure you that person's going to be at the top of the list of the inspector general when they talk to people, and we are currently reviewing our (disciplinary) options with respect to that person," McCormack said.
In Clinton's case, someone accessed her file last summer as part of a training session involving another State Department worker. McCormack said the violation was immediately recognized and the person was admonished.
The department's internal computer system "flags" certain records, including those of high-profile people, to tip off supervisors when someone tries to view the records without an appropriate reason.
McCormack said information on the incidents points to workers' "imprudent curiosity" more than something more sinister.
But "we are not dismissive of any other possibility, and that's the reason why we have an investigation under way," he said.
Former Independent Counsel Joseph diGenova, who investigated the 1992 scandal, said the firings of the two contract employees will make the investigation more difficult because the inspector general can't compel them to talk.
"My guess is if he tries to talk to them now, in all likelihood they will take the Fifth," diGenova said, referring to the Fifth Amendment's protection against self-incrimination.
Likewise, Patrick Kennedy, the top management official at the State Department, briefed the candidates' staffs on Capitol Hill, then said to reporters, "The State Department has very, very rigorous rules about controls and access for privacy material. We review them regularly and we have a large organization with a lot of people in it. Mistakes and errors happen from time to time. ... We caught these and we've got to work and correct that process."
Attorney General Michael Mukasey said the case has not yet been referred to the Justice Department for investigation, and indicated prosecutors were likely to wait until the State Department's inspector general concludes that inquiry. But Mukasey did not rule out the possibility of the Justice Department taking an independent look.
"Have they asked us to become involved - no," Mukasey said. "When, as, and if we have a basis for an investigation, including a reference - that is, one basis would be a reference - we could conduct one."
Asked what another basis could be, Mukasey said: "I don't want to speculate but if somebody walked in here with a box full of evidence, they wouldn't be turned away."
McCormack declined to name the companies that employed the contractors, despite the urging of a senior House Democrat, Henry Waxman of California, who said such information is in the public interest.
"At this point, we just started an investigation," McCormack said. "We want to err on the side of caution."
Sen. McCain, who was in Paris on Friday, said any breach of passport privacy deserves action.
"The United States of America values everyone's privacy, and corrective action should be taken," McCain said.
It is not clear whether the employees saw anything other than the basic personal data such as name, citizenship, age, Social Security number and place of birth, which is required when a person fills out a passport application.
The file also includes date and place of birth, address at time of application and the countries the person has traveled to.
According to a report in Computer World, the system also stores investigative reports that might have been compiled in connection with granting or denying a passport or in connection with any violation of passport criminal statutes. (Read more about passport applications in the CBSNews.com campaign blog Horserace)
"It is worth noting that that earlier situation (in 1992) also was characterized as isolated and nonpolitical when the news initially emerged," said Howard Berman, D-Calif., chairman of the House Foreign Affairs Committee.
"This time, as then, Congress will pay close attention to the depth of executive branch involvement in the rifling of presidential candidates' passport files," he added.
The Washington Times first reported the incident involving Obama.