Kristin Cavallari Shares Sweet Flashback Photo of Her Missing Brother, Thanks Fans for Prayers and Support

Kristin Cavallari sent a sweet message to her Instagram and Twitter followers expressing her gratitude after news broke that her brother, Michael Cavallari, has been missing for 10 days.

"I appreciate the thoughts and prayers that have been extended to me and my family," the 28-year-old former MTV star wrote. "I have such gratitude for those that have shared their support."

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On Instagram, the message accompanied a throwback photo of Kristin and her brother as kids.

The Grand County Sheriff's Office tells ET that Michael, 30, was last seen on security cameras at a convenience store in Monticello, Utah. A 2014 Honda Civic registered in his name was found abandoned about 100 miles north of the shop in Grand County on Nov. 27.

Law enforcement responded to a call about the vehicle that was found in a rural part of Utah just west of the Colorado border, about 225 miles from Salt Lake City. Upon arriving at the scene, police recovered credit card transaction receipts in the car from the aforementioned store, which led investigators to view the security footage and positively identify Michael as the driver.

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In 2004, Michael appeared in a scene of Laguna Beach with his younger sister. He now lives in San Clemente, California, while Kristin and her husband, NFL pro Jay Cutler, reside in Nashville, Tennessee with their three children -- 3-year-old Camden, 1-year-old Jaxon and Saylor James, who was born late last month. The Cavallari family has ties to the Utah-Colorado area, as Kristin was born in Denver.

Anyone with information or knowledge of the whereabouts of Michael is asked to contact the Grand County Sheriff's Office at 435-259-8115 or visit their website.

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