The FBI has expanded its investigation into former Secretary of State Hillary Clinton's emails, with the bureau now looking into whether she violated federal laws by making "materially false" statements in writing, orally or through a third party, reports Fox News.

Intelligence sources speaking under condition of anonymity told Fox News that federal agents are looking into the possibility that the Democratic presidential front-runner violated U.S. Code 18, Section 1001.

The law makes it illegal for any federal employee in the executive, legislative or judicial branch of the U.S. government to "knowingly and willfully" make a statement or entry that "falsifies, conceals, or covers up by any trick, scheme, or device a material fact," make "any materially false, fictitious, or fraudulent statement or representation," or make or use "any false writing or document knowing the same to contain any materially false, fictitious, or fraudulent statement or entry."

Violation of the code is a felony offense that can be punishable by five years in prison.

Clinton, her aides or attorney could be charged under Section 1001 if they were not forthcoming with the FBI about her emails, their classification or whether only non-government messages were destroyed - even if the statements were not made under oath, the source told Fox News. "The agents involved are under a lot of pressure and are busting ass," the source said.

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

Judge Andrew Napolitano noted that Martha Stewart was charged under the same section.

Two other government sources told Fox News that the FBI is conducting its own classification review of Clinton's emails, "effectively cutting out what has become a grinding process at the State Department."

Clinton also faces a criminal probe related to the Espionage Act. The "gross negligence" language of the law makes it a crime to store classified national defense documents on unsecured systems, even if doing so was unintentional, reports The Daily Mail.

Meanwhile, a new McClatchy/Marist poll found that 68 percent of Americans believe Clinton acted either unethically or illegally by exclusively using a home-based private server during her time as the nation's top diplomat.