Besides the dead and injured among 25 people crammed into the sport utility vehicle that collided with a semi-trailer, 19 others were found huddled near a second SUV that caught fire in the same area, officials say.

The 19 were detained by US Customs and Border Protection agents, CBP spokesman Macario Mora said on Wednesday.

The dozen survivors from the collision were all taken to hospitals for treatment.

Both the Ford Expedition involved in the crash and Chevrolet Suburban that inexplicably burst into flames appeared to be the same vehicles seen in surveillance footage leaving the vicinity of the fence breach about 48km to the east, Mora said.

While Border Patrol agents regularly scout the area for illegal crossings, neither vehicle had been under pursuit by US authorities beforehand, according to the agency.

New details of Tuesday's fatal highway wreck near the remote farming community of Holtville, about 16 km north of the border, came to light as the CBP mounted an investigation into the suspected human smuggling operation believed to have led to the tragedy.

"Human smugglers have proven time and again they have little regard for human life," the agency said in a statement.

Of the 13 people who died in the collision, at least 10 were Mexican nationals, and Mexicans were also among the injured, according to Mexico's foreign ministry.

Guatemalan authorities said they had preliminary information that one Guatemalan national died and another was injured.

Mora said investigators believe both carloads of migrants crossed into California through a gap cut out of the border fence by smugglers in an area known for frequent illegal crossings.

That section of the fence was installed in 2009, some eight years before the Trump administration took office calling for a border wall.

Australian Associated Press