Verified answer

This is from Gone from season 7. Basically this Canadian girl disappeared on a class trip to New York and it turns out these three boys were suspected of her rape, murder, and kidnapping. The one boy (the one Casey was hugging) was Jason. His cousin had pretty much forced him into attacking the girl and it turned out that Jason was the only one who DIDN'T rape her. Rather, they had consensual sex in the backseat of the car and then Nicky and Doug raped her. Her body was dumped and when Jason went to court to testify against them and an indictment came back against the other boys, Casey tried to comfort him by hugging him because he was extremely upset. he is later kidnapped and dumped in the Hudson, but his body gets caught on the bridge. Casey and Cragen stand there looking at the body being pulled out and Casey blames herself; she also blames herself for his disappearance, period. It is revealed that Nick was sleeping with a young court officer and she let him use her car to dump Jason's body; Nick bought her a new car to cover it up, but the detectives figured it out and the boys are arrested. We cut to black with Casey walking triumphantly out of the room. Hope I helped :)