Police say Adams-Kinard, 16, and Demetrius Glenn, also 16, beat Belton to death last week as he was waiting outside a Spokane Eagles Ice-A-Rena for a friend. Police have said the motive for the crime was robbery.
Belton, 88, was shot in the leg during the Battle of Okinawa, one of the fiercest battles of the war.
The teens are charged with first-degree murder and first-degree robbery. Glenn turned himself in Thursday, and Adams-Kinard was arrested early Monday in a basement apartment in Spokane after police followed up on tips from his family.
The note was found on a bed at the home where Adams-Kinard was arrested, police said, reports the station. Court documents reportedly said the author of the note detailed "going to get a zip of crack" from Belton and "socking" him three times. It also described robbing Belton after the attack.
The signature on the note, which was reportedly intended for the suspect's mother, appeared to match Adams-Kinard's, according to the station.
In court Tuesday, Adams-Kinard's lawyerprompting the attack. Investigators told KREM the claim was "ridiculous."
Belton's beating death has drawn national outrage and widespread media coverage. The veteran will be laid to rest with full military honors Thursday, followed by a church service on Friday.
The charges against Adams-Kinard and Glenn carry a potential life sentence.