A fifth “Game of Thrones” prequel series is officially in the works at HBO, Variety has confirmed.

Bryan Cogman, who has worked on “Game of Thrones” since Season 1, has been tapped to pen a potential fifth prequel project for the network. It was previously known that four prequels were in development, though George R.R. Martin said that a fifth was also in the works. When Martin first spoke of the fifth writer being recruited in May, he said that apart from himself and “Thrones” experts Elio Garcia and Linda Antonsson, “I don’t know anyone who knows and loves Westeros as well as he does.”

Cogman has worked on the show in various capacities over the years, most recently as a writer and co-executive producer. Among the episodes Cogman has written for the series are “Stormborn,” the second episode of Season 7, and “The Broken Man, the seventh episode of Season 6 which introduced the character of Lyanna Mormont.

The four writers previously announced are Max Borenstein (“Kong: Skull Island”), Jane Goldman (“Kingsman: The Golden Circle”), Brian Helgeland (“Legend”), and Carly Wray (“Mad Men,” “The Leftovers”).

Details for the five projects are being kept under tight guard, though Martin has made clear that each project is a prequel rather than a spinoff and that none of the current main cast of the series will be involved. He has also ruled out fan theories that any of them will focus on his characters Ser Duncan the Tall and Aegon the Unlikely, or Dunk and Egg as they are popularly known. He has also said that Robert’s Rebellion will not be the subject of any of the prequels.

Entertainment Weekly first reported this news.