Keyes was a suspect in several kidnappings and murders at the time he killed himself in a jail cell.
The notes were found "beneath Keyes’ body" after his death, the FBI said.
"Unreadable and covered in blood, the notes were sent to the FBI Laboratory in Quantico, Va., where they were enhanced to a more legible condition for analysis and review," the FBI said. "There was no hidden code or message in the writings, and no other investigative clues as to the identity of other possible victims were found. The notes remain difficult to read in parts, but the FBI is not planning on releasing a 'translation' and will not offer any commentary on the meaning of the writings."
A federal grand jury indictment of Keyes was handed down in April 2012, charging he "kidnapped and killed 18-year-old Samantha Koenig."
"Based upon investigation conducted following his arrest in Texas during March 2012, Israel Keyes is believed to have committed multiple kidnappings and murders across the United States between 2001 and March 2012," the FBI's division in Dallas said in January.