Newly released court documents said officers who arrested 16-year-old suspect Kenan Adams-Kinard at a home early Monday also found what appeared to be a confession letter signed by the youth.
The letter claims Adams-Kinard called Belton and arranged to buy crack cocaine from him, KHQ-TV reported. The letter also contends Adams-Kinard and the other 16-year-old suspected of the crime punched Belton three times before taking his wallet and more drugs from his pockets.
Spokane police have said there is no evidence that Belton was a drug dealer.
Meanwhile, the 88-year-old Belton, who was wounded in the Battle for Okinawa, will be buried with full military honors Thursday at Greenwood Memorial Terrace in Spokane. The public is invited.
Belton, who was known as "Shorty" because he was not much more than 5 feet tall, was found beaten in his vehicle outside an Eagles Lodge on the evening of Aug. 21. He died the next day of his injuries.
The two 16-year-olds - Adams-Kinard and Demetrius L. Glenn - have been charged with first-degree murder and first-degree robbery. They will be tried as adults and face up to life in prison if convicted.
The Associated Press does not generally identify minors accused of a crime but is naming the teens because of the severity of the charges.
According to the court documents released Wednesday, the author of the letter claims that he called "Shorty" and they agreed to meet to make a drug deal.
Feeling he was shorted in the deal, the letter writer says he "socked" Belton.
"I took his wallet and another ounce of crack from his pockets. He was unconscious so I made sure he was still breathing, and then I took off," the court documents quote from the letter.
Belton was born in Sunnyside, Wash., and raised in Spokane. He survived being shot in the leg in 1945 at Okinawa, one of the fiercest battles of the war, and went on to spend 33 years working for Kaiser Aluminum before retiring in 1982.