The investigation now includes Wake, Durham, Pitt, Craven, Johnston, Chatham, Randolph, Surry and Edgecombe counties.
ABC11 first reported on the issue in multiple Wake County high schools February 18.
The SBI says if parents or students have tips to share that will help in the investigation, they can be emailed to Ops-Center@ncdoj.gov.
Law enforcement is asking parents to talk to their children about the dangers of posing for or posting sexually explicit photos online and warning students about the consequences.
So far, no arrests have been made in the case. Distributing these types of pictures of minors is a felony and could result in up to a year of jail time.