The baby, Joseline Eichelberger, was found Sunday at about 4:30 p.m., Calverton Park police officer Chris Robertson told ABC News.
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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.
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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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