The bus' driver, Shuang Qing Feng, lost control and hitting a concrete median along the Pennsylvania Turnpike around 3:40 a.m. Sunday, National Transportation Safety Board official Jennifer Homendy said.
The bus shot up an embankment, flipped on its side and then back into the barrier, at which point a FedEx truck slammed into the bus's undercarriage, she said.
Feng, of Queens, N.Y., Eileen Zelis Aria, 35, of The Bronx, and Jaremy Vazquez, 9, of Brooklyn, were ejected and killed, officials said.
Pennsylvania residents Dennis Kehler, 48, and Daniel Kepner, 53, were killed with their UPS tractor-trailer slammed into the bus in the chain-reaction crash that ensued.
Nearly 60 people were injured in the crash, not including those in the second UPS truck or Mercedes Benz, reports say.
The bus had departed Queens late Saturday night before stopping in Manhattan and New Jersey. It was headed for Ohio and Kentucky.
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