Officers found the remains in Brackley Park, Hull, ten days ago.
Cops are thought to believe the victim may been hit by a train from the nearby rail line which carries freight from Hull docks.
But police now think the incident, in the eastern area of the East Yorkshire city, may have happened months or even several years ago.
It is unknown if cops have been able to identify the victim, HullLive reports.
The investigation is being led by British Transport Police.
Police were originally called to the scene at 6pm on Wednesday December 7 following “reports of human remains found”.
On Friday there were four scenes of crime tents within a cordon in the park.
Crime scene investigators in white forensic overalls were seen at the site, sparking concerns among locals.
But because the remains have been there for some time it is thought the scene may have been disrupted by animals and wildlife.
Nicola, a mum who lives next to the park said locals have been left “shocked” by the incident.
She told HullLive: “It is a lovely place for families around here but the area has had its trouble before, especially in the summer.”
On Thursday, British Transport Police said: “We’re still searching the area.
“It’s believed to be the remains of one person – and detectives are following a number of enquiries to identify them.”