HOLTVILLE, CALIF. -- The 13 people killed in one of the deadliest highway crashes involving migrants sneaking into the U.S. had entered California through a section of border fence with Mexico that was cut away, apparently by smugglers, immigration officials said Wednesday.

Surveillance video showed a Ford Expedition and Chevrolet Suburban drive through the opening early Tuesday, said Gregory Bovino, the Border Patrol's El Centro sector chief. The video has not been publicly released because it's part of an ongoing investigation.

The Suburban carried 19 people, and it caught fire for unknown reasons on a nearby interstate after entering the U.S. All escaped the vehicle and were taken into custody by Border Patrol agents.

The Expedition crammed with 25 people continued on, and a tractor-trailer struck it a short time later. Ten of the 13 killed in that crash have been identified as Mexican citizens. The Border Patrol said its agents were not pursuing the vehicle before the wreck.

The opening in the fence was about 30 miles (48 kilometres) east of the crash in the heart of California's Imperial Valley, a major farming region now at the height of a harvest that provides much of the lettuce, onions, broccoli and winter vegetables to U.S. supermarkets.

It was made of steel bollards that were built before former President Donald Trump blanketed much of the border with taller barriers that go deeper into the ground. Photos show a panel of eight steel poles was lifted out and left on the ground in the desert next to an old tire and other debris.

"Human smugglers have proven time and again they have little regard for human life," Bovino said. "Those who may be contemplating crossing the border illegally should pause to think of the dangers that all too often end in tragedy, tragedies our Border Patrol Agents and first responders are unfortunately very familiar with."

The breach occurred in a busy area for illegal crossings near the Imperial Sand Dunes where migrants often climb over an aging barrier and wait for drivers to pick them up, hoping to avoid the scrutiny of Border Patrol agents at checkpoints on highways leading to Los Angeles, San Diego and Phoenix.

A pandemic-related measure that allows the Border Patrol to expel people without an opportunity to seek asylum potentially leads some to try to evade authorities instead of surrendering, sometimes with fatal consequences. The U.S. Centers for Disease Control and Prevention introduced expulsion powers nearly a year ago under Trump, and the Biden administration has signalled no plans to lift them anytime soon.

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

Seats in the SUV 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 crash happened in an area that became a major route for illegal border crossings in the late 1990s after heightened enforcement in San Diego pushed migrants to more remote areas.

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 Trump. His administration's first wall project was in Calexico.

When police arrived Tuesday at the crash site about 125 miles (200 kilometres) 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.

The men and women in the SUV ranged in age from 15 to 53, and those who survived had injuries that were minor to severe, including fractures and head trauma, 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, suffered moderate injuries.

The crash occurred around 6:15 a.m. under a clear, sunny sky at an intersection just outside the community of Holtville, about 11 miles (18 kilometres) 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.

It's not clear if the SUV ran the stop sign or had stopped before entering the highway. How fast both vehicles were going also wasn't yet known.

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.

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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.