UPDATE 1-Clinton, McCain passport records were also viewed

(adds comments from State Department spokesman)

WASHINGTON, March 21 (Reuters) - The State Department acknowledged on Friday that workers had snooped into passport files of Sens. Hillary Clinton and John McCain, a day after disclosing workers had improperly viewed the file of Sen. Barack Obama.

The department apologized to the three top U.S. presidential candidates -- Democrats Clinton and Obama and Republican McCain -- and promised an investigation into the breach.

Clinton's file was accessed last summer by a trainee hired to help relieve the backlog of passport applications at the time, said State Department spokesman Sean McCormack.

He said the department also detected earlier this year that McCain's passport file was accessed by one of the same people who looked at Obama's file.

The incident is an embarrassment to the Bush administration and revived memories of the controversy that erupted in 1992 after State Department officials searched former President Bill Clinton's passport and citizenship files when he was the Democratic presidential candidate.

Earlier on Friday, Rice told reporters she spoke with Obama about the breach. "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 and therefore, I will stay on top of it and get to the bottom of it," she said.

McCormack later said she had called Clinton and would speak with McCain, who was traveling in Europe.

"Secretary Rice expressed the same sentiments to Senator Clinton, and I would expect that she would say the same thing to Senator McCain: We're sorry that this happened and ... We're going to do a full investigation," he said.

McCain, who was traveling in Europe, told reporters he did not have much to say on the matter "because I was over here and I only know what I saw. But if anyone's privacy is breached they deserve an apology and a full investigation and I believe that will take place."

On Thursday, the State Department said it had fired two employees and disciplined a third for having snooped in the passport files of Obama, an Illinois Democrat.

(Writing by Arshad Mohammed, editing by Jackie Frank and Vicki Allen)