Also publicly sworn in for a second term was Vice President Joe Biden, who, like Obama, also took his official oath Sunday.

CNN: Sworn in again, Obama lingers for last look

"What they're trying to find out through this investigation is what government official broke the oath that they signed to protect classified information, " said Matthew Miller, who left the Justice Department in 2011 and is now a public affairs consultant in private practice.

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Every U.S. government official swears a solemn oath to uphold and defend the Constitution.

CENTERFORSECURITYPOLICY: The other debate: Frank Gaffney vs. Suhail Khan

We both were found to have lied, not under oath, but in official proceedings and were reprimanded.

CNN: Transcript: Opening statement of Rep. Frank

Secretary Napolitano, thank you for administering the oath and making it official.

WHITEHOUSE: President Obama Speaks at a Naturalization Ceremony

Two of the most serious charges against him, of making a false official statement, and of lying under oath and obstructing justice, were dropped on a technicality.

BBC: Q&A: Iraq prison abuse scandal

Thank you, Secretary Napolitano, for being here to administer the oath --for making it official.

WHITEHOUSE: Naturalization Ceremony for U.S. Service Members

Previous to the execution of any official act of the President the Constitution requires an oath of office.

CNN: Sen. Allard's closed-door impeachment statement

Francisco Lopes, briefly appointed governor of the central bank at the time of Brazil 's devaluation in January, refused to testify under oath to a Senate inquiry into leaks of official information.

ECONOMIST: Kosovo

It wasn't until the 1990s that Rhodes-Hughes was asked whether she would ever be willing to testify under oath -- an invitation coming not from a prosecutor or law enforcement official but from author Philip H.

CNN: STORY HIGHLIGHTS