UPDATE: E! News has learned that Kristin Cavallari's brother was arrested days before his disappearance, on Nov. 23, in Dana Point, Calif. While the mother of three has yet to comment on the latest news, she thanked fans for their support via Instagram Monday.
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The search is on for Michael Cavallari who is believed to be the brother of the reality star and fashion designer.
According to a press release shared by the Grand County Sheriff's Office, Michael's 2014 Honda Civic was found abandoned Nov. 27 on I-70 in Utah.
A search of the immediate area by authorities did not produce the driver. The vehicle, however, came back registered to an individual in San Clemente, Calif., which is a city in Orange County.
Credit card transaction receipts from Monticello, Utah also pointed police to security footage from a convenience store where the driver of the vehicle was positively identified as Michael.
E! News has reached out to Kristin's team for comment. The former Hills star and her husband Jay Cutler have yet to publicly comment on the disappearance.
Just last month, the couple welcomed their third child and first daughter into the world.
In fact, Kristin had to miss her ten-year high school reunion in Laguna Beach, Calif. just last week to care for her newborn baby.
The Grand County Sheriff's Office is investigating the situation as an active missing person case. If anyone has information on Michael's whereabouts, they are asked to contact the office at (435) 259-8115 or visit their website.
(Originally published on Sunday, Dec. 6, 2015, at 3:37 p.m. PDT.)