By Alexandra Meeks and Ray Sanchez, CNN

(CNN) -- The 13 people killed when an SUV was struck by a semitruck in California's Imperial Valley are believed to be among dozens of undocumented migrants who entered the United States through a hole in the border fence with Mexico, the US Border Patrol said Wednesday.

At least 44 people had managed early Tuesday to enter the US through a 10-foot breach in the border fence near Interstate 8, the Border Patrol said in a statement.

Surveillance footage reviewed by border agents shortly after 6 a.m. showed two separate vehicles leaving the area where the fence was breached in what officials are investigating as "smuggling events."

One vehicle, a maroon Ford Expedition, was carrying 25 people when it was struck just after 7 a.m. by the semitruck towing two trailers near Holtville, according to officials

At least 10 of the 13 killed in the crash were Mexican nationals, according to consular officials.

Separately on Tuesday morning, Border agents responded to a call of a red Chevrolet Suburban engulfed in flames near the intersection of Interstate 8 and State Route 115, where they found 19 people hiding in the brush and determined they had entered the country through the breached border fence, the CBP said.

"Human smugglers have proven time and again they have little regard for human life," the CBP's El Centro sector chief, Gregory Bovino, said in the statement. "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 CBP said it is investigating the "smuggling events" and offered no other details "pending potential prosecution."

Cause of the crash is still unknown

It's unknown what caused the semitruck to ram into the side of the Ford Expedition at an intersection on a rural road, 10 miles north of the Mexican border.

"It would be premature of me to speculate on what happened at this collision. The important thing is that 13 people died in this crash," said California Highway Patrol Border Division Chief Omar Watson. "We owe it to the families of those that were killed and injured as well at the public to conduct a complete and thorough investigation."

The California Highway Patrol is leading the accident investigation.

CHP officials did not immediately respond to a CNN request for comment.

US Customs and Border Protection said agents never attempted to stop or pursue either vehicle.

Who are the victims?

Hospital and police officials on Tuesday gave slightly different tallies of the injured and dead in the incident.

Judy Cruz, the managing director of the emergency room department at El Centro Regional Medical Center, said the vehicle had 28 occupants and that 15 had died.

But Watson said at a morning briefing there were 25 occupants in the Expedition and that 13 died, including the driver of the SUV. The passengers in the SUV ranged in age from 15 to 53, he said.

It isn't yet known where all of the SUV passengers were from, as some didn't have identification, Watson said, though he did add that the driver was from the Mexican city of Mexicali.

"We are close to the border, so we have people who come back and forth daily for work," Watson said.

Mexican consulate officials said they was saddened by the news and are working to confirm the nationalities of the other three passengers.

The semitruck driver was identified as Joe Beltran, a 68-year-old resident of El Centro, California. The California Highway Patrol said Beltran "sustained major injuries."

The trailers he was pulling were not loaded, Watson said Tuesday.

What happened

According to authorities, police received a call about the crash at about 6:15 a.m. At the time of the crash, the semitruck was traveling north on State Road 115 and the 1997 Expedition was traveling west on Norrish Road in the Holtville area, approximately 100 miles east of San Diego, Watson said.

The semitruck, a 2011 Peterbilt, was traveling at an unknown speed when it struck the left side of the SUV.

Watson said it is unclear whether the SUV had stopped at an intersection but had entered the intersection "in front of the big rig."

A Ford Expedition typically can safely seat up to eight people. But this vehicle didn't have rear seats, Watson said.

"I don't know if they were cut out or removed, I'm not sure, but they were not in the vehicle," Watson said. "What it indicates is that there were not enough safety belts for the passengers."

The scene was gruesome, with some occupants ejected from the vehicle and killed, while others were found dead within the SUV, Watson said. He said 12 people died at the collision site and one person died at a hospital.

What's next in the investigation

The injured occupants were taken to El Centro Regional Medical Center (ECRMC), Desert Regional Medical Center in Palm Springs and Pioneers Memorial Hospital in nearby Brawley. UC San Diego Health said it received three patients transferred from ECRMC via air ambulance. CHP later said four people were transferred to UC San Diego.

Three of the four patients being treated at Desert Regional Medical Center were in the intensive care unit, hospital director of communications Todd Burke told CNN.

Three patients were taken to Pioneers Memorial, and two of them were later transported by air ambulance to Scripps Mercy Hospital in San Diego, CHP said.

At least one of the injured passengers from the SUV has been released from the hospital, Watson said.

US authorities are working with the Mexican consulate to determine who was in the SUV and notify their next of kin.

The National Transportation Safety Board said it will investigate the crash.

"NTSB, in concert with CHP, is conducting a safety investigation of Tuesday's fatal crash involving an SUV and a truck near Holtville, California," it said in a statement via Twitter.