A total of 23 people, from Guatemala, Honduras and Mexico and all in the country illegally, had been crammed into the cab and the bed of the pickup truck when the driver lost control around 6:30 p.m. Sunday. The truck, a white 2000 Ford F-250 Super-Duty extended cab pickup, slammed into the trees on the side of the road off northbound U.S. Highway 59, about 10 miles southwest of Goliad, officials said.

Eleven men, one woman and two young girls died. Nine others who were injured remain in local hospitals, state and federal authorities said.

It was unclear why the driver veered off the road, but a spokesman for the Texas Department of Public Safety, the state's top law enforcement agency, said a crash reconstruction team was investigating whether tire failure might have been a cause.

The crash was one of the deadliest single-vehicle accidents in South Texas, where a series of crashes involving vehicles filled with illegal immigrants have claimed numerous lives in recent months and years.

In April, a minivan crashed and rolled over near McAllen, killing nine people inside. The vehicle had been filled with 16 illegal immigrants, all of whom had been picked up from a so-called stash house and were being transported to their next destination when Border Patrol agents pulled it over.

The smugglers of illegal immigrants often use the highways and roads to travel from the border to Houston, Dallas and other cities north, or to head from the border to a South Texas stash house — an apartment or rental home where they wait for the next leg of their journey.

"There have been a number of accidents," Gregory Palmore, a spokesman for Immigration and Customs Enforcement's Homeland Security Investigations, which was assisting in the investigation, said of fatal crashes involving illegal immigrants. "Even if there's one fatality, there's more than there should be."

State officials said none of the 14 victims in the Goliad accident had been identified, and it was too early in the investigation to say where the truck was heading or where it had been coming from. The driver was among those who died.

At the site of the crash, about 145 miles from the Mexican border, there are two northbound lanes of the highway and one southbound lane. Investigators marked the truck's tire tracks with bright orange paint, highlighting its path as it veered from the northbound passing lane and down a ditch to the line of trees at the side of the road.

When state troopers and sheriff's deputies arrived at the scene Sunday evening, only six people were found in the truck; the rest had been ejected. "I've been here stationed in Victoria for 36 years, and this is one of the worst ones I've seen," said Trooper Gerald Bryant, who works in the nearby Victoria office of the Department of Public Safety.

By Monday afternoon, the truck had been removed from the base of the two thick trees, which were scuffed but still standing. The crash occurred on a straight stretch of highway in the daylight in dry weather.

Though illegal immigration at the United States-Mexico border has declined in recent years, the South Texas area has seen an increase in activity.

In the Customs and Border Protection's Rio Grande Valley sector, which includes the site of the Goliad crash, more than 2,000 illegal immigrants were apprehended this fiscal year as of May in nearly 80 stash houses, up from 1,012 in 69 stash houses in all of the 2011 fiscal year.