Authorities believe a Tennessee teenager is being "held against her will" and has grown to be in increasing danger after her tech-savvy father kidnapped her more than a year ago.

Daphne Westbrook has been missing since she never returned home from a weekend with her father in October 2019. Nearly a year and a half later, the 17-year-old’s dad, John Oliver Westbrook, has been indicted and charged with aggravated kidnapping -- but investigators have to find him and his daughter first, the Hamilton County District Attorney’s Office announced earlier this week.

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Daphne’s two dogs, Strawberry and Fern, also disappeared at the time.

"Daphne is being held against her will, unable to communicate with the outside world. Our investigation shows she is endangered both physically and mentally," the office said Tuesday. She "is increasingly at serious risk of physical and emotional danger."

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Investigators believe Daphne is either in Colorado -- where she and her father have been seen or spent time in the Denver and Pueblo areas in recent weeks -- or New Mexico, where Daphne’s Bible was discovered outside a Trader Joe’s. The office said "investigative leads" have shown John Oliver "is frequently shopping in Santa Fe stores."

Police said John Oliver Westbrook is an "IT expert" who specializes in blockchain technology, security and bitcoin.

"He is communicating in a way that’s almost impossible to trace, which makes it extremely difficult for investigators to generate significant leads in this case," the office said. Police believe he is making money doing computer work for small businesses.

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Authorities added: "We believe Westbrook does occasionally take Daphne to ride horses so we appeal to the New Mexico horsing community to be alert."

Police are asking anyone with information to call 1-423-209-7415 or email FindingDaphne@hcdatn.org.

If you've got a tip, send it to Stephanie.Pagones@foxnews.com.