He said there were 16 people who had refused to talk to investigators.
He called the case a tragedy made even worse because the victim was revictimized through social media.
Back-to-school parties turned ugly
Mays and Richmond were accused of raping the girl during a series of end-of-summer parties in August 2012.
According to prosecutors, each of them penetrated the victim's vagina with his fingers, an act that constitutes rape under Ohio law if it is not consensual.
Attorneys for the two boys had said they were not guilty.
CNN's policy is not to identify alleged victims of sexual assault. CNN is not naming the minors who have testified but is identifying Mays and Richmond, whose names have been used by court officials, their attorneys and in multiple media accounts.
At the heart of the case was the question of whether the victim was too drunk on the night of August 11 and the early morning of August 12 to understand what was happening to her and to consent.
The victim testified Saturday that she remembered little about the night because she was drunk.
During closing statements on Saturday, attorneys for the two boys argued the state failed to prove beyond a reasonable doubt that their clients raped the girl, calling into question the victim's credibility.
They also questioned whether an avalanche of cell pictures and videos and social media posts available in the days after the rape, as well as national media coverage ahead of the trial, tainted testimony.
But prosecutors told the judge there is no question the girl was "substantially impaired."
"The things that made her an imperfect witness -- that she doesn't remember a lot -- made her in every sense of the word a perfect victim," prosecutor Marianne Hemmeter said.
Victim says she remembers little from the night in question
The girl testified Saturday that she remembered drinking at the first big party of the night and then holding Mays' hand as she left with him, Richmond and others.
The next thing she remembers, she told the court, is waking up in the morning naked on a couch in an unfamiliar house. She covered herself with a blanket while she looked for her clothes. She testified she could not find her underwear, earrings or cell phone.
She testified she was "too embarrassed to ask what happened that night because I didn't remember."
The girl told the court she had a flashback memory of throwing up in a street somewhere sometime after she left the first party.
The victim was the 28th and final witness in a trial that has shone an unwelcome spotlight on Steubenville, a down-on-its-luck town of 18,400 residents along the Ohio River, and the Steubenville High School football team known locally as "Big Red."
A community divided
Critics have accused community leaders of trying to paper over rampant misconduct by players of the powerhouse "Big Red" football team and have suggested that other students took part in the assaults or failed to do enough to stop them.
During closing arguments Saturday, Hemmeter cast aside the outside attention that Anonymous helped bring to the case.
"This case isn't about a YouTube video. This case isn't about social media. This case isn't about Big Red football," she told the judge.
"This case is about a 16-year-old girl who was taken advantage of, toyed with and humiliated. And it's time people who did this to her are held responsible."
Earlier in the day, attorneys for Mays and Richmond challenged the credibility of the victim, calling two of the 16-year-old girl's former best friends to testify.