HOLTVILLE, Calif. (AP) — Investigators are looking into the possibility that migrants had breached a wall with Mexico before an SUV carrying 25 people collided with a tractor-trailer, killing 13 of them in one of the deadliest border crashes on record, an official said Wednesday.

Investigators believe other migrants crossed at the same time in an apparent smuggling operation but were not in the vehicle that crashed in the remote California desert, according to a U.S. official with direct knowledge of the investigation who spoke on the condition of anonymity because details were not intended to be public.

Seats in the 1997 Ford Expedition had been removed except for those for the driver and front passenger, said Omar Watson, chief of the California Highway Patrol's border division.

The cause of Tuesday's collision wasn't yet known, authorities said. The SUV is built to hold eight people safely, but smugglers are known to pack people into vehicles in extremely unsafe conditions to maximize their profits.

The Mexican government said 10 of the dead were Mexican citizens and that the nationalities of the three others was yet known.

The crash happened during the height of the harvest in California's Imperial Valley agricultural region, which provides much of the lettuce, onions, broccoli and winter vegetables to U.S. supermarkets. The community of Holtville where the crash occurred is a no-stoplight town with a gazebo in its large central square and calls itself the world's carrot capital.

The area became a major route for illegal border crossings in the late 1990s after heightened enforcement in San Diego pushed migrants to more remote areas. Many crossed the All-American Canal, an aqueduct that runs along the border and unleashes Colorado River water to farms through a vast network of canals.

Barely a mile from the crash, there is a cemetery with rows of unmarked bricks that is a burial ground for migrants who died crossing the border.

In 2001, John Hunter founded Water Station, a volunteer group that leaves jugs of water in giant plastic drums for dehydrated migrants.

“I was trying to figure out how to stop the deaths,” said Hunter, whose brother Duncan strongly advocated for border wall construction as a congressman.

Illegal crossings in the area fell sharply in the mid-2000s but the area has remained a draw for migrants and was a priority for wall construction under former President Donald Trump. His administration's first wall project was in Calexico.

U.S. Immigration and Customs Enforcement said late Tuesday that agents in its Homeland Security Investigations unit "have initiated a human smuggling investigation (into Tuesday's crash). The investigation is ongoing and no further details are available at this time.”

When police arrived at the crash site about 125 miles (200 kilometers) east of San Diego, some passengers were trying to crawl out of the crumpled SUV. Others were wandering around the nearby fields. The big rig’s front end was pushed into the SUV’s left side and two empty trailers were jackknifed behind it.

“It was a pretty chaotic scene,” Watson said.

The Border Patrol said its agents were not pursuing the vehicle before the crash.

Passengers in the SUV ranged in age from 15 to 53 and were a mix of men and women, officials said. The 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 the nearby California community of 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 and released.

The crash occurred around 6:15 a.m. under a clear, sunny sky at an intersection just outside Holtville, about 11 miles (18 kilometers) north of the border. Authorities said the tractor-trailer was heading north on a highway when the SUV pulled in front of it from a road with a stop sign.

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.

It’s not clear if the SUV ran a stop sign or had stopped before entering the highway. Speeds also weren't yet known.

The speed limit for tractor-trailers on the highway is 55 mph (89 kph), according to California Highway Patrol Officer Jake Sanchez. The other road also has a 55 mph speed limit.

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, taxing the brakes and making it tougher to steer the vehicle, said Frank Borris, former head of the National Highway Traffic Safety Administration’s Office of Defects Investigation.

“You’re going to have extended stopping distances, delayed reactions to steering inputs and potential overreaction to any type of high-speed lane change,” said Borris, who now runs a safety consulting business.

SUVs of that age tended to be top-heavy even when not carrying a lot of weight, Borris said.

“With all of that payload above the vehicle’s center of gravity, it’s going to make it even more unstable,” he said.

The crash happened amid farms that grow a wide variety of vegetables and alfalfa used for cattle feed.

Many workers commute cross the border from Mexico to California during the winter harvest, taking buses and SUVs to the fields from the small California city of Calexico just before dawn.

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Associated Press reporters Stefanie Dazio in Los Angeles, Julie Watson in San Diego, Anita Snow in Phoenix, Tom Krisher in Detroit and Mark Stevenson in Mexico City contributed.