Benjamin was the son of Lisa Marie Presley and grandson of the late Elvis Presley.
Earlier in the week, the Los Angeles County Medical Examiner's Office ruled that his official cause of death was a shotgun wound and the manner of death was ruled suicide.
Riley's post continued, “There are no words for you. Angel is the closest I could think of. Pure light. Baby brother. Best friend. Wild man. Intellectual. Witness to my life. Twin soul. Protector. Too sensitive for this harsh world. I hope you give me strength to endure the giant hole you’ve left in my heart. I hope you give me the strength to eat. I hope you’re cradled in love. I hope you feel my love. I hope you feel god. You are god. I can’t believe you’ve left me.”
Riley and Benjamin share a father in musician Danny Keough. Lisa Marie and Danny divorced in 1994. Benjamin also has 11-year-old twin sisters, Finley and Harper Lockwood, whose father is Lisa Marie's ex-husband Michael Lockwood.
The actress concluded her heartfelt post, writing, "Not you sweet Ben Ben. Anyone but you. I guess this is true heartbreak. I hope we meet again."
See her full tribute here.