Singh said prosecutors plan to appeal to a higher court and seek the death penalty for the three defendants who received life sentences.
Thousands of members of a radical Hindu group had demanded the release of the defendants, insisting they were innocent.
The trial was shifted to Pathankot, a town in neighboring Punjab state, following accusations that local Hindu leaders and politicians were trying to block the investigation.
The prosecution said the girl was raped in a small village temple in Kathua district after having been kept sedated for four days, and was then bludgeoned to death.
The girl's father, Mohammed Akhtar, told The Associated Press by phone that the men should be "punished speedily, not just convicted."
"Our family has gone through hell," he said. "Our hearts are bleeding. These beasts should be hanged."
India has been shaken by a series of sexual assaults in recent years, including the gang rape and murder of a student on a New Delhi bus in 2012. That attack galvanised a country where widespread violence against women had long been quietly accepted.
While the government has passed a series of laws increasing punishment for rape, it's rare for more than a few weeks to pass without another brutal sexual assault being reported.
- AP