No suspicious circumstances surrounded the death of seven-year-old Conley Thompson, whose body was found on a building site, police said.
The youngster died when he got stuck in a vertical plastic pipe and his clothing restricted his breathing on the site in Worsbrough, near Barnsley.
South Yorkshire Police said there was "no suggestion of any third party involvement" but they are investigating the cause of the boy's death and working with the Health and Safety Executive.
Conley vanished after playing with friends in Locke Park, a mile away from the site and about two miles away from his home in Barnsley, police said.
He had gone to play at about 6pm on Sunday and was due to return home around two hours later, but never turned up.
South Yorkshire Police said officers had checked the building site in the hours of darkness after Conley was reported missing on Sunday night, but they found it to be securely locked with a large perimeter fence.
Detective Chief Inspector Dave Stopford, the senior investigating officer, said: "The results of the post-mortem suggest that Conley had, by some means, entered a vertical pipe on a nearby construction site and was unable to get out.
"Conley's weight caused him to fall further into the pipe and pushed his clothing up around his body restricting his breathing.
"There is no suggestion of any third party involvement."
He added: "Our job is to now piece together Conley's movements on Sunday evening and establish how he accessed the construction site.
"Forming part of that investigation we will continue to work with the Health and Safety Executive and our thoughts remain with the family and friends of Conley at this extremely difficult time."
Conley's grandmother, Deborah Fraser, said she was "shell-shocked" by his death.
"As a person he was a lovely little lad, boisterous, got into anything and everything but his smile could light up a room," she said.