The reality TV star's sibling was last seen on Nov. 27, according to reports.
Kristin Cavallari's brother, Michael Cavallari, has been missing for ten days, an officer from the Grand County Sheriff's Office tells ET.
According to the sheriff, Michael 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.
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Law enforcement responded to a call about the vehicle that was approximately 100 feet off Fly Wash Road, which is located roughly five miles south of Interstate 70. The area is in a rural part of Utah just west of the Colorado border, about 225 miles from Salt Lake City.
When police arrived at the scene, they found 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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Michael lives in San Clemente, California, and is the brother-in-law of NFL pro Jay Cutler. His sister, Kristin, married the Chicago Bears quarterback in June 2013, and the two share three children together -- 3-year-old Camden, 1-year-old Jaxon and Saylor James, who was born late last month. The 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.