A pickup truck overloaded with illegal immigrants veered off a highway and crashed into trees in rural south Texas, killing 14 people and leaving nine injured, authorities said.

U.S. Immigration and Customs Enforcement Homeland Security agents were looking into the human smuggling aspect of the case.

A Texas Department of Public Safety accident reconstruction team meanwhile investigated the cause of the Sunday evening crash in Goliad County, about 240 kilometres northeast of the border with Mexico.

The pickup crammed with 23 immigrants from Mexico and Central America crashed less than an hour's drive from the site of the nation's most deadly immigrant smuggling case, where 19 immigrants died in 2003 after being placed in a sweltering trailer.

"This is the most people I've seen in any passenger vehicle, and I've been an officer for 38 years," Texas Department of Public Safety spokesman Gerald Bryant said.

The driver was among the 11 found dead at the scene, Bryant said, adding that investigators were trying to confirm his name.

Six of those who died in the crash were still inside the cab of the mangled vehicle and one more remained in the truck's bed when emergency crews arrived at the scene, Bryant said. Others were scattered on the roadway and in a ditch between the pavement and the fence line where the truck stopped. Bryant said he saw at least two young children among the dead.

There was very little in the way of belongings or identification, he said.

Suspected human trafficking

The truck was registered in Houston to someone other than the driver, Bryant said. A woman who answered at the address where the truck was registered said she was the daughter of the man listed as the owner. But she said they had sold the truck and it was no longer theirs. Asked when the sale took place, she hung up.

A DPS accident reconstruction team was investigating the crash, but Bryant said it could be another week or two before its work concluded. U.S. Immigration and Customs Enforcement agents were working to confirm the identities of the victims and investigate the possibility that they had been smuggled into the United States.

ICE spokesman Greg Palmore said that among the 11 men and three females who died were citizens of Mexico, Guatemala and Honduras.

Victor Corzo, the head of legal affairs at the Mexican Consulate in San Antonio, said the only Mexican citizen involved was a 22-year-old man from Tamaulipas state. The state in northern Mexico borders south Texas.

Corzo said Monday the consulate was still working to notify the man's family.

The white 2000 Ford F-250 pickup was heading north on U.S. 59 when it drove off the right side of the highway near the unincorporated community of Berclair and struck two large trees, Bryant said. Berclair is about 150 kilometres southeast of San Antonio.

It is not uncommon for human traffickers to try to maximize profits by overloading vehicles with illegal immigrants as they head north from the Texas-Mexico border. In April, nine Mexican immigrants died near the border when the teenage driver of their van crashed after fleeing Border Patrol. There were 18 people in that minivan.

In that case, six adults face a variety of federal charges and the 15-year-old driver was charged in state court with nine counts of murder.

Bryant told The Associated Press that several of the survivors of Sunday's crash had life-threatening injuries. He did not have their official conditions but described them as "very serious." The injured were taken to hospitals in San Antonio, Victoria and Corpus Christi.