The baby, Joseline Eichelberger, was found Sunday at about 4:30 p.m., Calverton Park police officer Chris Robertson told ABC News.
The car had been left outside a home in Calverton Park, about 14 miles outside of St. Louis. The temperature climbed to 79 degrees in St. Louis that day.
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No arrests have been made, Robertson said. However, police spoke with two undisclosed persons of interest.
Police are working with the St. Louis County prosecutor's office as the ongoing investigation moves forward, he said.
Hot car deaths reached a record level last year with at least 52 children killed, from California to Tennessee to Mississippi, according to national nonprofit KidsAndCars.org.
"I feel like a piece of my heart is gone because she was a part of me," said Joseline's cousin, Lilly Bellfield.
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