Connecticut state police are seeking witnesses as investigators try to determine the cause of a crash that killed four people on Interstate 95 in Stamford.

All four victims were in an SUV that rear-ended a tractor-trailer that had slowed for heavy traffic in the northbound lanes shortly after 3 a.m. Saturday, state police said. The tractor-trailer driver was not injured, authorities said.

It was not clear why there was heavy traffic early Saturday morning. State police said Sunday that all circumstances about the accident remained under investigation. According to the state Department of Transportation, lane closures on the opposite side of I-95 in Stamford were scheduled due to construction at the time of the crash.

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State police listed the same address in Philadelphia for all four victims: Joel Contreras-Paniagua, 49, Lorena Contreras Francisco Reyes, 59, Joel Contreras Francisco, 29, and Daniel Contreras Francisco, 23. Troopers did not specify their relationships in an initial report. No one else was in the SUV.

The 45-year-old tractor-trailer driver is from Elmwood Park, New Jersey.