Convicted killer Samuel Little -- seen in this undated photo obtained on November 28, 2018 from the Ector County Sheriff's Office in Texas -- has confessed to 90 murders. [Ector County Sheriff's Office/AFP/File / HO]
US investigators have so far confirmed that a 78-year-old drifter -- who could potentially be the most prolific serial killer in American history -- is responsible for more than 40 murders, authorities said Thursday.