18-year-old Kody Patten was arrested for the murder. Now his fiancee, Toni Fratto, has also been arrested on a charge of open murder.
During a hearing on Tuesday to admit Patten into a Nevada facility for a mental evaluation, his attorney stated that Fratto had contacted his office on April 22nd with information on the case.
Defense attorney John Ohlson told the court that the interview with Fratto was recorded, and submitted the recording as evidence. Ohlson said, "She conceded that she was present during the killing and said that she participated in the killing and gave us specific details."
The Elko County Sheriff's Office says Fratto stated she used a deadly weapon in the crime and was involved in the destruction and disposal of Costanzo's personal property and potential evidence.
After reviewing the new evidence, detectives decided there was enough probable cause to arrest Fratto.
Attorneys say it's too soon to say how this new development will impact the case. Patten could face the death penalty.
Meanwhile, Costanzo's mother said she was surprised to learn about a second accomplice.
"It was very difficult to find out about kody but then to now find out about Toni as well, it's still shocking tot he family," said Celia Costanzo. "It's overwhelming.
Fratto, like Patten was to graduate in a few weeks from West Wendover High School in Nevada. Costanzo said she did know of Fratto.
"I don't know what possessed Toni to come forward as she did I don't know," Costanzo said. "I'm grateful that she has. It helps."
At the high school students were just getting used to the idea of a classmate accused of murdering another classmate. But to learn of a second classmate involved has left many of the students in shock. But others feel betrayed by Fratto.
Sandra Vera was in the same computer class as Fratto and said she kept to herself following Patten's arrest.
"She told everybody back then that she didn't have anything to with it (murder)," Vera said. "After that she kind of kept to herself, an outcast. You never see her walk with anybody."
Vera said there were a few students who attempted to befriend her and would talk with her.
"They tried to talk to her and make her feel better," she said. "A lot of those students are betrayed by this now."
As for Costanzo, she said they are bracing for the two trials and hopes that they will find justice in the coming year.
"I know there are some hard things yet to come with the court hearings and stuff," she said. "It will be difficult but we will get through it."