The circumstances surrounding the August death of a 6-month-old boy were "suspicious" but the manner of death could not be determined, according to an El Paso County coroner's report.

The baby's parents - Nicholas Manuel Lucero, 21, and Elizabeth Elishah Lopez, 20 - were arrested Thursday on suspicion of child abuse resulting in death after a grand jury indictment was filed Wednesday, court records show.

According to the coroner's report, the baby's death could be classified as sudden unexpected infant death, associated with an unsafe sleeping environment.

Colorado Springs police said the infant died Aug. 19 after he was placed in a closed shelving unit to sleep with pillows in front of the shelf to muffle the sound of the baby's crying.

The indictments also indicate that the baby was left in the care of a minor, but do not elaborate.

"An extensive investigation into Nicholas' death resulted in a number of varying accounts by family members in regard to the infant's body position as well as to the configuration of the pillows that surrounded him when he was discovered unresponsive on the floor of the closet," the coroner's report stated. "Needless to say, the circumstances surrounding his death are suspicious."

The autopsy report "may be amended" if more information becomes available, the report says.

Both parents remained in custody Monday at the El Paso County jail in lieu of $50,000 bonds, and are scheduled for first appearances in court on Thursday, records show.