DENVER (KDVR) -- On June 12, 1988, a woman's body was discovered in Weld County between Erie and Dacono.

Nearly 40 years later, officials are still searching for answers into what happened to 22-year-old Robin Denise Nelson.

Nelson on June 7, 1988, left her son in the care of a friend in Denver, according to the Weld County Sheriff's Office. She never returned.

The friend called Denver police, but a body was discovered June 12, 1988, at West County Road 8 and the Interstate 25 Frontage Road. The Colorado Bureau of Investigation identified the body, which was "in the advanced stages of decomposition," several days later.

The investigation into Nelson's whereabouts before her death did not turn up any potential suspects, but the sheriff's office said she was an acquaintance with Tammy Lynn Cheeks, another woman found dead in a Weld County irrigation ditch several years later. Cheeks' death is also unsolved, and authorities have not determined a cause of her death.

The coroner's office ruled Nelson's death was caused by a drug overdose, but the sheriff's office said "there are suspicious circumstances surrounding her death that need to be answered."

Anyone with information about Nelson's or Cheeks' death can call the Weld County tip line at 970-304-6464 or email [email protected].