The former fiancée of slain U.S. Ambassador Christopher Stevens told Newsmax TV she believes Hillary Clinton stepped down as secretary of state after realizing she had made a huge mistake in not paying closer attention to the growing threat of violence in Libya that led to the Benghazi attack.

"She was at the top at the time; she was the secretary of state," Lydie Denier said Tuesday on "Newsmax Prime" with J.D. Hayworth. "What I always wondered is suddenly she decided to quit her job as secretary of state earlier than people were expecting."

"Did someone tell her 'listen you made a big boo-boo and you have to get out?' That subject has never been brought to the public."

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Denier is author of the new book, "A Voice for Ambassador J. Christopher Stevens," which chronicles the life and times of Stevens, who tragically died with three other Americans on Sept. 11, 2012, and her dedicated quest to find out what really happened.

She told Hayworth she doesn't believe Clinton can be trusted with American national security because of her failure to prevent the bloodbath in Benghazi, despite warnings of growing tensions and the need for more security.

"If we cannot trust her for Americans, how can we trust her for the whole country?" Denier said. "It doesn't make sense. In my book, I do talk about that and about what happened . . . Chris was and knew, was extremely knowledgeable about Libya and what was going on. He should've been listened to."

The book also details her romantic relationship with Stevens.

"The Chris Stevens I knew was fun and loving, caring, always putting others in front of everything instead of himself," she said. "I had fantastic memories, friends have fantastic memories about him and there's no more. He's gone. He doesn't have a voice, and so I decided to be his voice.

"First, he was excited to become an ambassador because Chris always wanted to become an ambassador. It was a dream that finally came true and Libya he really, really loved Libya, the country, its people, its culture.

"Chris had been in Libya twice before, but since his last tour in 2011, he didn't realize that a lot of things changed and very quickly. Even in Tripoli . . . thugs actually were throwing stones at Chris. So, Chris was not allowed to run outside the compound. He had to stay inside."

She added: "What happened to Chris will never happen again. It cannot happen again. This was a big mistake and that mistake should never, never happen again."