The suspect, Marcel Hesse, was arrested at a fast food stall in the city of Herne, west of Dortmund, after a four-day manhunt.
The 19-year-old had been sought after claims he killed a nine-year-old neighbour in Herne and posted gruesome images of the slain child online.
When he was picked up, Hesse told police of a fire in a nearby apartment. Police found the body of a man there.
Prosecutor Paul Jansen said Hesse gave a statement to authorities after his arrest.
Earlier: A 19-year-old man has been arrested on suspicion of killing a nine-year-old boy in the German city of Herne and posting pictures of the victim's body on the internet, police said.
Officers made the arrest after the man called from a fast food stall and reported a fire in a nearby apartment, where police later found another male body, police spokesman Gunnar Wortmann said.
The suspect, named by police as Marcel Hesse, had been sought since the death of a neighbour's child on Monday.
The killing shocked the industrial city of Herne, not least because police said the suspect had boasted about his actions online.
Authorities said Hesse had posted a photo and video of the child online.
North Rhine-Westphalia's top security official, Interior Minister Ralf Jaeger, said at the time that the images "left even the investigators stunned".
Hesse was clearly identifiable, standing with bloody hands next to his victim, police said.
A large-scale manhunt was launched, with police using helicopters and sniffer dogs to search the area.
They warned the public not to approach the suspect, saying Hesse knew martial arts and could be armed and dangerous.