Well, if he's the JonBenet killer, he might have killed others. Makes sense to at least check, given his perverted background.
The note is signed "S.B.T.C" There is no period after the letter "C." When writing we end a thought by placing a period at the end of the sentence. Not using a period tells us the writer intentionally stopped writing. There may be conflict at this point in the story. The writer may have more information that was purposely withheld. There has been a lot of speculation as to what the letters S.B.T.C. mean. The one that makes some sense to me is "Saved By The Cross." This is because the Ramseys profess to have faith in God and because word "Victory" precedes the initials S.B.T.C. As all Christians know, it is through Christ's sacrifice on the cross that we have "victory" over death.