"State police say a preliminary investigation has found that a tractor trailer came up upon slow moving traffic and caused the crash," NBC Philadelphia reports.
Update at 5:24 p.m. ET Driver Charged
The New York Times is reporting that the truck driver involved in the crash has been charged:
The truck driver, identified as Kevin Roper, 35, of Jonesboro, Ga., was charged later on Saturday with one count of death by auto and four counts of assault by auto, according to Middlesex County's acting prosecutor and the state police.