When frozen bodies are found in Nevada floating in water, the BAU's investigation leads them to suspect that the victims were part of a cult that could be behind the deaths, in an episode entitled "The Forever People"
Reid: Huh. Morgan: What? You see something? Reid: Encryption's a highly specialized skill set but it's fundamentally a mathematical process, which means the human process sometimes through technique can reveal where you learned it. I think I know where he learned how to do this. Kate: Where? Reid: Harvard. Which oddly enough isn't known for its advanced math program but it is known for one particular class. When you're good at math - good enough to get into Harvard - you take a math class called "Math 15". When you're better than that you take "Math 25", but when you're the best, the absolute best, you take "Math 55": Honors Advanced Calculus and Linear Algebra. Graduates are immediately employed by the U.S. Government because they're too dangerous to work anywhere else. More specifically, they're employed at the NSA.