A day after an SUV carrying 25 people crashed into a big rig truck, leaving 13 dead and the rest injured, investigators are beginning to piece together what brought the vehicles to the highway intersection Tuesday morning.

Several agencies are descending Wednesday on Imperial County, including U.S. Immigration and Customs Enforcement, the National Transportation Safety Board and the California Highway Patrol. Their task will be to iron out the details of Tuesday’s horrific crash, and to answer the nagging question: Why were 25 people traveling in a hollowed-out SUV?

It is “too early in the investigation to say what they were doing or where they were coming from,” California Highway Patrol Chief Omar Watson said Tuesday. “It’s a very sad situation. That vehicle is not meant for that many people.”

Twenty-five people were crammed into the 1997 Ford Expedition, which had only driver and front passenger seats, with the rear seats removed. When the SUV entered the intersection of Highway 115 shortly after sunrise and collided with a big rig truck, several people were ejected from the vehicle into the road. Everybody involved was either killed or injured, Watson said.

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