The child, identified by police as Joe, was reported missing on June 17, leading to a huge search operation that was concluded after receiving a tip from a person who said they'd heard sounds beneath the street in Oldenburg, police said in a statement released on Tuesday (June 28) and obtained by NBC News on Wednesday (June 29) morning.
Officers located the boy under a manhole cover in the area last Saturday (June 25), eight days after he was initially reported missing.
"After evaluating all traces and clues, the officers now assume that Joe crawled through a drain into the rainwater sewer system on the day of his disappearance and lost his orientation after several meters," police said in their statement. "The police have ruled out third-party negligence at this point in time."
Police said the boy was found "completely undressed" but didn't have any serious injuries, however, was severely hypothermic and dehydrated.
A specialist company was employed by local authorities to search the sewer network between the boy's home and the area in which the child was found in order to find clues to how he disappeared.
Joe was reported to be seen on a property near a drain pipe with a diameter of 100 centimeters at around 6:00 p.m. prior to his disappearance on June 17.
"Investigators therefore consider it highly probable that Joe climbed into this concrete pipe while playing," police said in their statement. "It can now be assumed that Joe has increasingly lost his bearings in the system and could no longer find an exit."