It was officially confirmed in The Twilight Saga: The Official Illustrated Guide that Adam was killed by Victoria after discovering that the sun could not kill vampires, because she didn't want him to tell the other newborns.
Bree Tanner mentioned him randomly in The Short Second Life of Bree Tanner when Diego informed her that indirect sunlight doesn't harm vampires. She thought it was impossible and said, "But Shelly and Steve, Doug and Adam. That kid with the bright red hair. All of them. They're gone because they didn't get back in time. Riley saw the ashes."
|Seattle newborn army|
|Leaders||Victoria • Riley Biers|
|Group leaders||Raoul • Kristie|
|Members||Adam • Casey • Dean • Diego • Doug • Fred • Heather • Jen • Jim • Kevin • Logan • Sara • Shelly • Steve • Bree Tanner • Warren|