I spoke to a family friend who quit the police department I was dealing with over ethical concerns. He said that it was pretty standard practice, especially when dealing with a case they had zero suspects on. He said, "they'll bring you in, they'll tell you you failed, and they'll tell you that if you confess, they'll go easy on you because you're so young. It's not the results that matter on a polygraph, it's the answers you give. Don't lie, and don't incriminate yourself."
He was 100% correct. They brought me back to the station, into an interrogation room, and after about an hour of waiting, told me exactly what he said they would.
I lost all respect for law enforcement through that process.