The probe into new Hillary Clinton–related emails is a "very serious development," former Justice Department official David Rivkin told CNBC on Friday.
He believes the Federal Bureau of Investigation received information that puts into question its fundamental conclusions about not bringing a case against Clinton. The FBI investigated the Democratic nominee for her handling of classified information in connection with private email servers while secretary of state.
On Friday, FBI Director James Comey sent a letter to lawmakers stating it is looking into new emails related to Clinton. Those emails were discovered during an FBI investigation in which the devices of former Congressman Anthony Weiner and Clinton aide Huma Abedin were seized, NBC News confirmed. The news was first reported by The New York Times.
"It clearly is something that requires them to take additional investigatory steps. It's not something that is sort of self-contained and can be just looked at and put to the side. So I think it's extremely serious," Rivkin said in an interview with CNBC's "Closing Bell."
He believes there are only two possibilities — that the new information shows that someone lied during the investigation, or that highly classified emails were compromised in ways that bring into question the FBI's decision not to recommend charges against Clinton.
"I don't think there can be anything else that would have caused Director Comey this close to the election to take this very important step, which of course has tremendous symbolic significance," said Rivkin, who also served as White House counsel to President Ronald Reagan and President George H.W. Bush.
In fact, the FBI and Justice Department both have a longstanding policy of being "extremely reluctant to take any public investigatory steps this close to the election," Rivkin noted.
Hillary Clinton's campaign has demanded the release of those emails.
"The Director owes it to the American people to immediately provide the full details of what he is now examining. We are confident this will not produce any conclusions different from the one the FBI reached in July," campaign chair John Podesta said in a statement.
— CNBC's Christine Wang contributed to this report.