The 3-year-old boy found in a La Plata park died of dehydration and hypothermia, or low body temperature, said Diane Richardson, spokeswoman for the Charles County Sheriff's Office. The Office of the Chief Medical Examiner in Baltimore ruled his death a homicide last week.
Bruce Goldfarb, spokesman for the medical examiner, said Tuesday that he couldn't talk about the case.
Ji'Aire Lee and his 24-year-old mother arrived at Wills Memorial Park the morning of May 20, Richardson said. The mother put the boy, who was alive, into the swing, where he stayed until he and his mother were found the morning of May 22.
One side of the park is bordered by woods in La Plata, a town about 30 miles southeast of Washington, with a population of about 8,700, according to the town's website. The park is in view of a few houses.
Sheriff's deputies went to the park about 7 a.m. on May 22 after being called to check on the welfare of the woman and child, Richardson said. The officers went to remove the boy from the swing and give him first aid, but "it was instantaneously clear the child was dead," she said. There were no signs of trauma to his body.
May 21 was cold for late May, Richardson said, "rather chilly and it drizzled all night." According to the National Weather Service, the temperature at Dulles Airport outside of Washington ranged from 58 to 50 degrees on May 21. The low temperature was three degrees below normal.
The mother was taken to a hospital after she and the boy were found.
The child's maternal grandmother told FOX 5 that her daughter had been diagnosed with a mental illness a few months ago and had been seeking help. She didn't give specifics.
The Charles County State's Attorney's Office will review all the information deputies have gathered during the investigation and "will make a decision in terms of charges."
The Associated Press contributed to this report.