The Blades endured a torrid afternoon on the pitch, suffering a heavy defeat that extended their winless run in the Premier League to six games.
According to The Shields Gazette, Sheffield United are now under investigation regarding a supporter incident.
Projectiles were thrown on the pitch as the Magpies celebrated their third goal and one is said to have struck the foot of midfielder Elliot Anderson.
Newcastle boss Eddie Howe discussed the incident after the game, telling reporters: “Elliot hasn’t spoken to me but I did see things thrown from the stand which isn’t what we want to see.
“My thought initially was just for the welfare of the players and hopefully everyone has come through okay.”
FA rules state clubs have a responsibility to ensure spectators conduct themselves “in an orderly fashion” and do not throw missiles or potentially harmful objects on the pitch.