Investigators with the National Transportation Safety Board are expected to arrive in Southern California on Wednesday to probe the previous day's fatal crash involving an SUV that was said to be carrying 25 people near the U.S.-Mexico border, according to officials. Thirteen people died in the crash.

The NTSB and the California Highway Patrol will begin their investigation into the Tuesday morning crash near Holtville, California. At least three NTSB agents are expected to scour the scene, with the department’s investigator-in-charge expected to arrive first, the agency said. It was not immediately clear what time they were expected to be on scene.

The crash occurred around 6:15 a.m. at an intersection just outside Holtville, which dubs itself the world’s "carrot capital" and is about 11 miles from the U.S.-Mexico border. Authorities said a tractor-trailer and its two empty containers were northbound on State Highway 115 when a Ford Expedition, which had everything except for the front and driver seats removed, pulled in front of it from Norrish Road.

On Wednesday morning both vehicles had been cleared from the roadway, which had been reopened, said Officer Jake Sanchez, a CHP spokesperson.

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A California Highway Patrol report said the SUV entered an intersection directly in front of the big rig, which hit the left side of the SUV. Both vehicles came to a halt on a dirt shoulder.

Twelve people were found dead when first responders reached the two-lane highway. Another person died at a hospital, California Highway Patrol Chief Omar Watson said.

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It's not clear if the SUV ran a stop sign or had stopped before entering the highway. It's also not yet known how fast the tractor-trailer was traveling.

The speed limit for tractor-trailers on the highway is 55 mph, according to CHP spokesperson Sanchez. The other road is also 55 mph.

 A 1997 Ford Expedition can carry a maximum payload of 2,000 pounds. If it had 25 people inside, that would easily exceed the payload limit, which taxes the brakes and makes it tougher to steer, said Frank Borris, former head of the National Highway Traffic Safety Administration’s Office of Defects Investigation.

The cause of the collision was so far unclear, authorities said, and it also was not immediately known why so many people were crammed into a vehicle built to hold eight people safely.

U.S. Immigration and Customs Enforcement (ICE) announced later Tuesday that agents have opened a human smuggling investigation.

Most of the dead were Mexican, an official said, though their immigration statuses were not known as of Wednesday morning.

It wasn't immediately clear whether the SUV was carrying migrants who had crossed the border, ferrying farmworkers to fields, or was being used for some other purpose.

"Special agents from Homeland Security Investigations San Diego responded ... and have initiated a human smuggling investigation," the agency said in a statement, adding that other details weren't being released.

Macario Mora, a spokesman for Customs and Border Protection, said agents were not pursuing the SUV at the time of the crash, which was initially rumored.

"It was an unusual number of people in an SUV, but we don’t know who they were," Mora said.

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The people in the vehicle ranged in age from 15 to 53 and were a mix of males and females, officials said. The 28-year-old driver was from Mexicali, Mexico, just across the border, and was among those killed. The 68-year-old driver of the big rig, who is from nearby El Centro, was hospitalized with moderate injuries.

The passengers' injuries ranged from minor to severe and included fractures and head trauma. They were being cared for at several hospitals. One person was treated at a hospital and released.

Mora had no updates in the investigation as of 6 a.m. local time Wednesday.

Fox News' David Aaro and The Associated Press contributed to this report.