Green announced he was stepping down Sunday citing "major health and personal matters."
Monday, a spokesperson for the State Bureau of Investigation said the agency has been asked by Franklin County District Attorney Sam Currin to look into missing money at the sheriff's office.
Both the SBI and Currin were contacted by ABC11 Monday.
Currin told ABC11 that the investigation was a factor in Green's resignation, but declined to elaborate further.
Green was elected sheriff in 2006 and was re-elected in November.