Fratto, along with her boyfriend Kody Cree Patten, 18, is accused in the slaying of 16-year-old Micaela Costanzo, whose body was found in the desert west of the Utah-Nevada border last March.
Fratto might never have been charged, police said, except for her surprise confession to Patten's defense attorneys.
At a preliminary hearing last year, Springgate argued the veracity of Fratto's claims, recorded by Patten's attorneys and played in court, that the three drove to a spot five miles west of Wendover on March 3, where Patten and Fratto allegedly beat Micaela with a shovel, slashed her throat and buried her in a shallow grave.
Fratto's defense team had filed a motion to have that purported confession barred.
But as Fratto met with prosecutors in Elko County, Patten had also been involved in plea talks that would have led to him testifying against Fratto, Springgate said.
Those talks appear to have stopped for now and Patten still faces the possibility of death if convicted.
Patten's father also testified last year that Fratto had come to him to confess her role in the slaying, which prosecutors could have used against Fratto if the recording was suppressed.
Springgate said Fratto met with prosecutors last week and provided a "lengthy statement." She has also agreed to testify if called to do so at trial, Springgate said.