The formal cause of death was hyperthermia and environmental heat exposure, Maryland Office of the Chief Medical Examiner spokesman Bruce Goldfarb said on Tuesday. He said the autopsy reports won't be complete for at least a month.
Toxicology tests were negative, leaving investigators with no explanation for why an apparently healthy woman would sit with her small children in a hot car for hours until they died.
"It gives us more questions than answers as to why," said Hagerstown Police Capt. Paul Kifer. "It's very possible she was having a mental breakdown and didn't realize what she was doing would cause her death."
Guyanese immigrant Allison Keyonda Pluck, formerly Allison Lancaster, 32, was found dead in her silver Nissan sedan June 16 along with the bodies of Shania Gill, 18 months, and Shameka Gill, 6 months. The mother was reclined in the driver's seat and the girls were strapped in their child seats in the back with the windows closed, the ignition off and the doors unlocked, police said.
Police have found no connection between the family and the school. They say Pluck had quit her job at a Wal-Mart in Frederick, about 20 miles away, a month or two before she died.
The girls' father, construction worker Paul Gill, also a Guyanese national, said he was puzzled by the deaths. He and Pluck lived in apartments about a mile apart in Hagerstown, about 70 miles west of Baltimore, but he said he saw his daughters often.
"It's very hard," Gill said in a telephone interview.
He said he met Pluck at a function in West Virginia, where police say she lived before moving to Maryland.
Pluck's father, Lancaster Colbert of Georgetown, Guyana, said in a telephone interview that Pluck had left the South American country for the United States with her husband at least four years ago. The couple later separated, he said.
Lancaster said he didn't speak with his daughter much after she left Guyana - she usually didn't return his calls - but "she never told me anything, really, that was bad," he said.
Police say a Western Heights Middle School employee, leaving on the last day for teachers before summer break, spotted the bodies in the late afternoon on a day when outdoor temperatures peaked near 90 degrees.
Investigators learned that Pluck had purchased a DVD player at a local Wal-Mart at about 6 a.m. and then drove to the school, arriving at 6:16 a.m. Surveillance video showed her car driving around the back of the building and then parking in an employee lot, where it remained until the bodies were found around 4:30 p.m.