LA PLATA, Md. -- Further investigation and toxicology tests are needed to determine what caused the death of a Maryland child who was found dead in a park swing as his mother pushed him, authorities said.An autopsy has been completed on the 3-year-old boy found Friday in La Plata, Maryland, but the cause of death was still under investigation, Charles County Sheriff's Office spokeswoman Diane Richardson said Monday.The 24-year-old mother was taken to a hospital Friday. Richardson said the woman remained hospitalized Monday. Officials haven't identified her. They've said she may have been at the park for hours before the boy was discovered.Detectives were working to establish a timeline of the days before his death, Richardson said. Previously, she said there were no obvious signs of foul play but that it hadn't been ruled out.Also Monday, a woman who identified herself as the child's maternal grandmother, told a TV station her daughter had been diagnosed with a mental illness a few months ago and had been seeking help. She didn't give specifics.The woman, who identified herself as Vontasha Simms, also told WUSA-TV her daughter ordinarily "would never harm my grandson in any type of way."Police have not commented on details of the investigation or whether mental illness played a role.