Once Cattran opened up the pipes in the building, he discovered what he described to the Daily Mirror as “lumps of flesh the size of my fist and strips of flesh that looked as though they had been cut from an arm.” “I was trying to think what sort of animal it could be. As I was prodding around, I thought it’s obviously not a dog, there’s no fur. It’s not a chicken — not that much of it. It was all bruised up, and eventually I got to thinking that it had to be a body,” he explained.

Per Murderpedia, Nilsen, in a last-ditch attempt to escape detection, spent that night underneath the manhole, clearing the remains from the drains. However, this only raised suspicion when Cattran returned the next day with his boss to find the bulk of the remains suddenly gone. They alerted the authorities, who uncovered the remaining scraps of flesh, as well as human bone, that had been left in the drain. Once the pathologist David Bowen confirmed the remains were human, police returned to Cranley Gardens to arrest Nilsen. According to Biography, Nilsen readily confessed to the crimes, even directing detectives to garbage bags full of human body parts stored around his apartment. 

Dennis Nilsen was sentenced to life imprisonment on six counts of murder and two charges of attempted murder. He remained imprisoned at Full Sutton maximum security prison until his death in 2018, per Newsweek.