PUEBLO — A volunteer group discovered a large trash bag with human remains in Colorado near the Arkansas River, police said.

The Pueblo Police Department responded to the area around 10 a.m. Tuesday to investigate the discovery by a group cleaning the riverbank, The Pueblo Chieftain reports.

The group alerted a park ranger to the black bag found on a steep embankment south of the river’s bike trail, authorities said.

The Pueblo County coroner removed the remains from the scene and could not immediately make an identification.

The remains are believed to be those of an adult, Pueblo police Sgt. Franklyn Ortega said.

The remains were “completely decomposed” after exposure to the elements for weeks or months, Ortega said.

Police cordoned off the area as part of their investigation into how the remains were deposited near the river, Ortega said.