Democratic presidential candidate Hillary Clinton said Sunday that an e-mail released Thursday did not show she had mishandled classified information, the latest twist in questions about the former secretary of state's communications practices. 

"That did not happen, and it never would have happened," Clinton said in an interview on the CBS News broadcast, ''Face the Nation."" "That’s just not how I treated classified information." 

In a series of June 2011 e-mails, which the State Department released in response to a lawsuit, a top aide tells Clinton that there has been difficulty sending a piece of information over secure fax. "If they can't, turn into nonpaper with no identifying heading and send nonsecure," she wrote to the aide, Jake Sullivan, a former deputy national security adviser. 

The State Department has one system for transmitting classified information, and another for non-classified. Some communications contain both types of information.

"Oftentimes, there’s a lot of information that isn’t at all classified, so whatever information can be appropriately transmitted unclassified, often was. That’s true for every agency in the government and everybody who does business with the government," Clinton said. 

Clinton said Sullivan, who is now an adviser to her campaign, never sent the information, a set of talking points for an upcoming phone call. 

The former secretary of state used a private e-mail server to conduct government business while serving in President Barack Obama's Cabinet. The Federal Bureau of Investigation is looking into the safety of classified information under the set-up.

Clinton has said she did not send or receive information marked classified. 

State Department spokesman John Kirby said on Jan. 8 that Sullivan did not appear to have sent the talking points in question, and called the presence of unclassified information on a classified system "not uncommon." 

Clinton's interview marked a rare appearance on one of the Sunday political talk shows, unlike some candidates who appear regularly. Trump was scheduled to make three appearances on Sunday alone. 

During the interview, Clinton responded to a new Wall Street Journal/NBC/Marist poll which showed her neck-and-neck with Vermont Senator Bernie Sanders in Iowa and New Hampshire, which will hold the nation's first nominating contests in early February.

The poll showed Clinton leading Sanders by 48 percent to 45 percent in Iowa, within the margin of error. "These polls, they go up, they go down," she said. "I stay pretty focused."

Clinton, who has made support for gun control a key policy point, continued to criticize Sanders for his support for providing legal protection to gun manufacturers—a measure that the National Rifle Association supported. Sanders has tied his support for gun rights to his largely rural, low-crime state, among whose residents hunting is a popular pastime. 

In an appearance on ABC's "This Week with George Stephanopoulos," Sanders said he was "absolutely willing to take another look at that legislation and get rid of the onerous provisions.