Police say Douglas Anderson bludgeoned the 78-year-old Carpenter in the head and repeatedly stabbed him in the abdomen at Carpenter's home on July 8.
Investigators say they found Carpenter's blood in Anderson's vehicle and have video evidence that the vehicle was at the house at the time of the murder.
Police say they believe Anderson killed Carpenter to try to hide that he had defrauded him of about $30,000. Officials say Anderson had promised to use the money to buy cattle and hay for Carpenter but never went through with the transaction.
Investigators say Anderson also forged his wife's signature on a promissory note for $230,000 in which he again promised to buy cattle and hay.
ISP says Anderson was arrested Friday morning at his home in Salmon.
He is being charged with first degree murder, forgery, and three counts of grand theft.
He is being held in the Lemhi County Jail on $1 million bond.
He is expected to be arraigned in court on Monday.