What Has to Be Proved for Perjury Charges Against Hillary Clinton The questions that need answers to bring charges against the Democratic nominee.

 -- Two top Republicans sent a letter this week to the leading federal prosecutor in Washington, D.C. laying out their case for perjury charges against Democratic nominee Hillary Clinton over false testimony she gave to Congress.

The letter cites several inaccurate statements she made when testifying in October 2015 about her use of a private email server, and it notes that the FBI's own investigation appears "to directly contradict several aspects of her sworn testimony."

But is that enough to bring perjury charges? Here’s a look at what the law requires and how it affected two recent high-profile cases.

The Law

The letter sent by House Oversight and Government Reform Committee Chairman Jason Chaffetz, R-Utah, and House Judiciary Committee Chairman Bob Goodlatte, R-Virginia, specifically cites two U.S. laws governing charges of perjury and false statements.

The perjury statute (18 U.S.C. 1621) makes it a crime for anyone under oath to "willfully" make statements "which he does not believe to be true." The false-statements statute (18 U.S.C. 1001) makes it a crime for someone to "knowingly and willfully" provide false or fraudulent statements.

A third statute, not cited by the Republicans’ letter, similarly makes it a crime to "knowingly" provide false statements under oath, but specifically covers such statements before a grand jury or court.

A Successful Use Of Perjury Law

A federal jury in Washington, D.C. convicted Vice President Dick Cheney’s former top aide, I. Lewis "Scooter" Libby, of repeatedly lying about his role in leaking the identity of an undercover CIA operative.

Specifically, Libby told a grand jury and authorities that he couldn’t have been the source of the illegal leak because he first learned about the identity from NBC anchor Tim Russert in July 2003. But nine people came forward to say Libby talked with them about the operative's identity in the weeks before his supposed conversation with Russert. At trial, Libby claimed to have simply forgotten he actually learned about the identity from Cheney a month before he spoke with Russert.

Judge Reggie Walton told the jury that to convict Libby, prosecutors needed to prove more than just Libby's statements were false –- they needed to prove Libby "knew" his statements were false when he made them.

In closing arguments, prosecutors said the evidence showed Libby made a conscious decision to lie to the grand jury and FBI.

Libby "invents out of whole cloth, absolutely fabricates" his conversation with Russert, whose testimony at trial was "in direct contrast to what Mr. Libby claimed," prosecutors said.

And the notion that he would have forgotten about nine conversations with others, but recall his Russert conversation with "amazingly sharp and clear" details is "simply not credible," prosecutors told the jury.

Even Libby’s own defense team acknowledged that making false statements is not enough to convict Libby of the charges against him.

"There’s no question he got some things wrong in the grand jury, no question," defense attorney Ted Wells said. "But you've got to ask, 'Did he engage in intentional lying?'"

"There’s nothing wrong with circumstantial evidence," Wells added, "but it’s got to be powerful evidence, built on a strong foundation ... if you’re going to brand someone a criminal."

In March 2007, the jury convicted Libby on four of five counts, including false statements to authorities and perjury before a grand jury. President George W. Bush later commuted his sentence.

A Failed Use Of Perjury Law

Prosecutors failed to persuade a federal jury to convict baseball legend Barry Bonds on charges of lying to a grand jury.

Bonds was indicted over testimony he gave in 2003 to a federal grand jury, which was looking at the illegal distribution of steroid and human growth hormone to professional athletes and others, in San Francisco.

To protect his legacy, Bonds "chose to" lie when he told the grand jury he hadn’t knowingly taken steroids, and he "testified falsely when he was asked whether or not [his trainer] had given him human growth hormone," according to prosecutors in closing arguments at the 2011 trial.

"In order for the defendant to be found guilty," the judge presiding over the case told the jury, "the government must prove ... the defendant knew [his] testimony" was false.

"So what's the evidence that he knew he was taking steroids?" prosecutor Jeffrey Nedrow asked the jury, before offering his list of evidence: lab results showing the presence of steroids in Bonds' body, records showing Bonds complained of side effects consistent with steroids, witnesses who claimed to have talked with Bonds "for a number of years" about steroids and a recorded discussion about injections.

Nedrow added, "The question is: 'Is it possible to have these items in your body, these powerful drugs, and not know you have them in your body?'"

At least some of the jurors ultimately though it was possible, failing to reach consensus on any of the perjury charges against Bonds.

"The evidence that he did steroids might have been there, but 'Did he knowingly do steroids?' was the question. And we couldn't prove that beyond a reasonable doubt," one female juror told CNN at the time. "There was just a piece of the puzzle missing."

The jury ultimately convicted Bonds on one count of obstructing justice, but a federal appeals court threw out that conviction last year.

In June 2012, a federal jury completely acquitted another baseball legend, Roger Clemens, of charges that he committed perjury and lied to Congress about using steroids and performance-enhancing drugs. But in the Clemens case, prosecutors failed to prove beyond a reasonable doubt that Clemens' statements were in fact false –- the jurors therefore likely didn't have to consider whether he "knowingly" made false statements.

FBI Director Believes There is 'Not Evidence' to Prove Clinton Perjury

During his testimony before a House panel last month, FBI Director James Comey appeared skeptical that Clinton could be successfully prosecuted for perjury –- even as he acknowledged several of her public statements were false.

As the letter this week from Republicans states, "Contrary to her sworn testimony, Secretary Clinton sent and received emails that were marked classified at the time."

Comey told lawmakers that, in fact, three emails on Clinton's server had paragraphs "summariz[ing] something" and included a C in parentheses at the beginning of it, indicating the paragraph contained information "classified at the confidential level, which is the lowest level of classification."

But those paragraphs were "down in the body" of the emails, and "none of the emails had headers at the top of the document that said it's classified," Comey testified. Because of the improper absence of such headers, it "would be ... reasonable" for Clinton to think the emails did not contain classified information, or she may have missed the C markings buried in the tens of thousands of other emails exchanged, he said.

Even if she did notice the markings, "I think it's possible —- possible -— that she didn't understand what a C meant when she saw it in the body of the e-mail like that," he added.

"I don't think that our investigation established she was actually particularly sophisticated with respect to classified information and the levels and treatment," he testified. "One of the things I learned in this case is that the secretary may not have been as [technically] sophisticated as people assumed."

Consequently, Comey said, "There is, in my view, not evidence ... beyond a reasonable doubt that she knew she was receiving [or sending] classified information."

While some may believe a "reasonable person" in Clinton’s position must have known, "The 'reasonable person' test is not what you look at for perjury or false statements," Comey said.

The Republicans' letter this week laid out three other statements from Clinton -- made under oath -- that they say "are incompatible with the FBI’s findings." However, the letter does not address Clinton's intent at the time she made those statements.