Investigators recovered 18 bone fragments near an old chimney where investigators believe the body of a developmentally disabled 3-year-old foster child was burned, a county coroner said Thursday.

The boy's foster parents, charged with killing him, pleaded not guilty Thursday to murder.

The bone fragments were from a child 2 to 4 years old, said Dr. O'dell Owens, the Hamilton County coroner. Each was less than inch long and found in soil in rural Brown County, east of Cincinnati.

"The DNA that is being processed came from a small piece of tissue that was found," Owens said. Authorities have said some of the boy's remains may have been thrown into the Ohio River.

The foster parents, Liz and David Carroll Jr., are accused of wrapping young Marcus Fiesel in a blanket and packing tape and leaving him in a closet for two days while they went to a family reunion in Kentucky. The boy was dead when they returned, authorities said. A third adult who lived in the home has been cooperating with investigators.

The couple had reported Marcus missing on Aug. 15 and said he had disappeared from a public park. Hundreds of people joined a search for the child.

The Carrolls were arrested in Hamilton County Aug. 28.

Marcus' birth mother, Donna Trevino, filed a $5 million wrongful death lawsuit Tuesday against the Carrolls and government and private agencies involved in moving the boy to foster care. Marcus and two other children were removed from Trevino's home in April after allegations of neglect.