Police began searching for Marcel Hesse after the dead boy was found in his cellar on Monday night. They said he posted images of the body online.
He then reported a fire in a nearby flat, where another body was found.
Shocked locals have been gathering outside the apartment in Herne.
The nine-year-old boy died of multiple stab wounds, and the discovery of his body on Monday sparked a huge manhunt.
On Wednesday, with the suspect still at-large, children in Herne were told to play indoors at kindergartens. Police warned the public that the teenager may be armed.
Police said he shared images on the internet of himself next to the dead child.
Investigators in the nearby city of Bochum said the suspect had also spoken online of killing a woman, claiming she “put up more of a fight than the child” and weighed some 120kg (19 stone).
The motive for his alleged attack appeared to have been the woman’s bank, computer and phone details.