The culprit has been identified.
NFL Media's Mike Garafolo reports it was Patriots running back Rex Burkhead who collided with Tom Brady at practice, resulting in the thumb injury that has throw New England in a tizzy over the QB's readiness for the AFC Championship Game Sunday against the Jacksonville Jaguars at Gillette Stadium.
"Rex Burkhead was the running back," Garafolo said on "Good Morning Football." They were colliding as they were doing some sort of handoff in a little drill. I don't know exactly what happened, but it resulted in a cut.