Officials suspect that students could have stolen them as a prank or to use as morbid Christmas Ornaments, reports USA Today, but they currently have no leads.
The missing brains comprise half of the university's collection.
"We think somebody may have taken the brains, but we don't know at all for sure," psychology professor and co-curator of the collection, Tim Schallert, told the Austin American-Statesman.
Though the university was required to remove any data that might identify who the brain came from, it's believed that one of the missing brains belonged to notorious killer and University of Texas alumni, Charles Whitman. Whitman shot 46 people and killed 16 in a horrific massacre on the university's campus in 1966.
The University of Texas originally came to possess the brains after the Austin State Hospital transferred them to the school 28 years ago under a "temporary possession" agreement. However, because the school's psychology lab only had room for 100 brains, the remaining 100 were moved to the basement of the school's Animal Resources Center.
The University has launched an investigation to determine the circumstances of the brain collection's disappearance, and they have issued a statement saying they are "committed to treating the brain specimens with respect."
The remaining brains on campus have been moved to a new building and are being closely monitored by faculty.