The investigation on Friday was focusing on Karr's writings, including a 1982 high-school yearbook inscription he made for a friend.
Karr on Thursday said he was involved in the death of the child beauty pageant winner, whose body was found December 26, 1996, in the basement of the Ramsey family home in Boulder. However, questions have since been raised about some of his statements.
The yearbook entry ends with Karr writing in capital block letters, "Though, deep in the future, maybe I shall be the conquerer [sic] and live in multiple peace."
A ransom note found in the Ramsey home ended with the word "Victory!" and was signed "S.B.T.C." Authorities want to know whether those letters might stand for "shall be the conquerer" and whether they could have been written by the same person.
John Hargett, who once ran the documents section of the U.S. Secret Service, told CNN the letters represent "a very interesting coincidence." But he said he saw no similarity between the handwriting used in the ransom note and that in the yearbook.
Murder scene secrets
Under questioning by U.S. authorities in Bangkok, Karr has provided gruesome details about the condition of Ramsey's body -- information that had been kept secret for nearly a decade, a U.S. law enforcement source told CNN on Friday.
The details were known only to the medical examiner and investigators looking into JonBenet's slaying, the law enforcement official said. (Read about the crime scene)
Despite his remarks, Karr's ex-wife and family members insist he was not involved in Ramsey's death.
Some observers have speculated that the 41-year-old may have falsely confessed -- either intentionally out of a desire to link himself to such a high-profile case or because he is delusional.