Grand County, Utah, Sheriff's Office / . 1 / 7
Grand County, Utah, Sheriff's Office / . 1 / 7

Kristin Cavallari is thanking her friends and fans for their support as authorities continue to search for her brother who has gone missing.

Michael Cavallari's Honda Civic was found abandoned in remote Southeast Utah the day after Thanksgiving, but he hasn't been seen since and no one has has had contact with him. Police are reportedly treating it as an open missing person's case.

Kristin had remained mum about her brother and the search, but on Dec. 7 she broke her silence, taking to Instagram to post a throwback photo of her and her brother, along with a heartfelt thank you note to her 1.6 million followers.

"I appreciate all the concern, thoughts, and prayers that have been extended to me and my family," he captioned the photo showing both of them as young tykes on the beach. "We remain hopeful and I will share any news that we receive. I have such gratitude for all those that have shared their support."

Michael lives in San Clemente, Calif., and it's unknown why he was in that area of Utah, although it is a popular playground for outdoor recreational activities. His last known whereabouts were at a convenience store, about 100 miles from where his car was found.

Grand County Sheriff Steven White told the Today show, "There's no sign of foul play that we can tell. We're still piecing everything together."

Anyone with information regarding Michael should contact the Grand County Sheriff's Office at 435-359-8115.