Steubenville police Capt. Joel Walker said the Jefferson County girls, ages 15 and 16, were being held in juvenile detention.
The older girl was charged with aggravated menacing for a tweet that threatened homicide and said "you ripped my family apart," according to the attorney general's office. The girl is a cousin of defendant Ma'Lik Richmond, attorney general spokesman Dan Tierney said Tuesday.
A Twitter message from the younger girl threatened the accuser with bodily harm, leading to a menacing charge, the office said.
Such threats have to end, DeWine said Tuesday.
"People have the right to express their point of view, and they have the right to be stupid, and they have the right to be wrong, but they don't have the right under Ohio law to threaten to kill someone," he said.
The guilty verdict was barely an hour old Sunday when DeWine said he was continuing his investigation and would consider charges against anyone who failed to speak up after the attack last August. That group could include other teens, parents, school officials and coaches for the high school's beloved football team, which has won nine state championships.
A grand jury will meet in mid-April to consider evidence gathered by investigators from dozens of interviews, including with the football program's 27 coaches, which include junior high, freshman and volunteer coaches.
Text messages introduced at trial suggested the head coach was aware of the rape allegation early on. Reno Saccoccia "took care of it," defendant Trent Mays said in one text introduced by prosecutors.
DeWine said coaches are among officials required by state law to report suspected child abuse. Saccoccia has not commented.
Steubenville city manager Cathy Davison said residents want to see justice done, and the city will be better off going forward because of the wider investigation.
Steubenville schools Superintendent Mike McVey released a statement Monday reiterating his position that the district was waiting until the trial ended to take action. He declined to address the grand jury investigation.
It's unclear what could happen to the school's sports programs if coaches were charged. Sanctions against teams or programs typically involve violations of rules related to playing, such as improper recruiting of student-athletes or playing ineligible athletes, said Tim Stried, spokesman for the Ohio High School Athletic Association.
Mays and Ma'Lik Richmond were charged with penetrating the West Virginia girl with their fingers, first in the back seat of a moving car after a mostly underage drinking party on Aug. 11, and then in the basement of a house.
Mays, 17, and Richmond, 16, were sentenced to at least a year in juvenile prison for the rapes. Mays was ordered to serve an additional year for photographing the underage girl naked.
They can be held until they turn 21.
Special Judge Thomas Lipps recommended the boys be assigned to Lighthouse Youth Center-Paint Creek in Chillicothe which he said has a strong program for treating juvenile sex offenders.