Hillary Clinton listens to a question as she testifies before the House Select Committee on Benghazi, on Capitol Hill in Washington October 22, 2015. Reuters The FBI has reportedly expanded its probe into Hillary Clinton's private email server to examine whether "materially false statements" were ever provided to agents throughout the course of the case, Fox News reported on Thursday.

At the center of the new inquiry, according to Fox News, is US Code 18, Section 1001, which pertains to the willful falsification of material facts, statements, and documents during a federal investigation. The code is meant to penalize individuals who knowingly make false or misleading statements that waste federal agents' time and resources.

"This is a broad, brush statute that punishes individuals who are not direct and fulsome in their answers," former FBI agent Timothy Gill told Fox. "It is a cover-all. The problem for a defendant is when their statements cause the bureau to expend more time, energy, resources to deconflict their statements with the evidence."

Facing criticism earlier this year for exclusively using a private server during her time as US secretary of state, Clinton handed over about 30,000 work-related emails for the State Department to make public in March. She deleted about 31,000 more emails she says were personal in nature.

At the time, a US House of Representatives committee requested access to Clinton's server to ensure that she had not deleted any work-related emails. But her lawyer, David Kendall, told the committee that Clinton aides had changed the server's settings so that only emails she sent and received in the previous 60 days would be saved.

Bloomberg reported in late September that agents had been able to recover at least some of the 30,000 "private" emails Clinton deleted. Intriguingly, agents handed some of them over to investigators — indicating that they are relevant in at least some way to the FBI's ongoing probe.

Hillary Clinton testifying before the House Select Committee on Benghazi. AP

'A flawed process'

In August, Clinton handed her server over to the FBI, which has been looking into whether any classified information ever passed onto the server while she served as secretary of state.

As such, the probe has been centered around 18 US Code 793, a section of the Espionage Act related to gathering and transmitting national-defense information.

A couple of weeks into the investigation, Charles McCullough, the intelligence community inspector general, said that two emails had been found on Clinton's server from 2009 and 2011 that contained information regarded as "Top Secret/Sensitive Compartmented Information," one of the highest levels of classification.

But the intelligence community has since walked back on that claim.

"The initial determination was based on a flawed process," a source familiar with the probe told Politico's Josh Gerstein. "There was an intelligence product people thought [one of the emails] was based on, but that actually postdated the email in question."

U.S. Democratic presidential candidate Hillary Clinton listens to a question from the audience at the VFW Hall in Derry Thomson Reuters

After months of negative headlines that battered her campaign, the Democratic presidential front-runner apologized in September for her email arrangement. But she has insisted that she didn't violate protocol or pass along material marked classified.

Clinton's use of the server was allowed under State Department regulations, and, so far, her claim that she never sent nor received information marked classified has held up under scrutiny. Though some of her emails did contain classified or "top secret" information, they were only marked as such after they passed through her server.

Investigators are also examining what measures Clinton and her team took to secure the server, since there are rules governing how it should be configured so it is not vulnerable to cyberattacks.

So far, reports that hackers in China, South Korea, Germany, and Russia tried to break into her server — which was monitored by a relatively unknown tech firm with no security clearance — have not helped the 2016 presidential candidate wave away concerns.

"The fact that Clinton chose to use her personal email instead of a .gov account shows that she obviously doesn't understand security," Joe Loomis, CEO of CyberSponse, a software company, told Business Insider last month. "What she did is like inviting spies over to dinner — every device connected to the internet is an opportunity for them to collect intelligence."

FBI Director James Comey. Thomson Reuters

'A foul taste in the FBI's mouth'

In an interview with "60 Minutes" in October, US President Barack Obama said that though it was probably a "mistake" for Clinton to use a private email server during her time as secretary of state, it "is not a situation in which America's national security was endangered."

His comments reportedly angered the FBI, which perceived Obama's comments as an attempt to influence the outcome of their probe, The New York Times reported last month.

"Injecting politics into what is supposed to be a fact-finding inquiry leaves a foul taste in the F.B.I.'s mouth and makes them fear that no matter what they find, the Justice Department will take the president's signal and not bring a case," Ron Hosko, a former senior FBI official who retired in 2014, told The Times.

US Sen. Bernie Sanders (I-Vermont), Clinton's top rival for the Democratic nomination, had made a similar effort to write off concerns surrounding Clinton's emails a week prior to Obama's "60 Minutes" appearance.

During a Democratic presidential debate, Sanders said that he and the American public were "sick and tired of hearing about your damn emails!" Sanders has since backed off such strong language, saying last week, "There is an investigation. The FBI is doing what it's doing."

Hillary Clinton and Bernie Sanders. AP

But investigators are annoyed that the president and others passed judgment about whether Clinton's email setup endangered national security when officials have yet to determine whether her server was compromised by foreign adversaries.

Alex McGeorge, a cybersecurity expert at Immunity, believes that continued concern over Clinton's email setup is warranted.

"Clearly there's a problem with folks in Washington not taking cyber security seriously and it has serious consequences," McGeorge told Business Insider in an email.

He added: "So I think there is a benefit to riding this out until the bitter end with the hope that every other politician who witnesses the pain Clinton now has to go to will reflect on that prior to making security decisions."