NEW BRAUNFELS, Texas 芒鈧珺次元官网网址� Authorities in two Texas counties said they received phone calls about a pickup driving erratically shortly before a head-on collision between a truck and church bus in southwest Texas that killed 13 people returning from a retreat.

A traffic accident reconstruction specialist meanwhile suggested to The San Antonio Express-B次元官网网址 that any seatbelts fitted in the minibus might not have been suitable for protecting older, more vulnerable passengers, even if it was travelling at a moderate speed.

One man called the dispatch line just past noon Wednesday to report that a white Dodge pickup was swerving on the road, Uvalde police Lt. Daniel Rodriguez said Thursday.

"(The caller) was scared (the pickup driver) was going to cause an accident and asked us to send deputies," Rodriguez said. "Deputies were dispatched, but before they could reach the area, the same caller called 911 to report that the truck had been in an accident."

Dispatchers in Real County received a call from a woman who reported a truck was driving erratically on U.S. 83, county Constable Nathan Johnson said. Real County officials called Uvalde County officials to co-ordinate a response to send deputies. Then, the woman called back and said the truck that had been driving erratically had struck another vehicle before reaching Real County, Johnson said.

"Unfortunately, he struck a motor vehicle before anyone could respond," he said.

The wreck occurred along a curve in the road where the speed limit is 65 mph, DPS Sgt. Orlando Moreno said.

The Texas Department of Public Safety refused to speculate on the cause of the collision near the town of Concan, about 80 miles west of San Antonio, although one spokesman said the truck driver appeared to have crossed the centre line.

The National Transportation Safety Board sent investigators on Thursday to start looking into the crash. The agency did not make any statements on the investigation.

It is not yet clear if the bus passengers, aged 61 to 87, were wearing seatbelts or even if the minibus was fitted with suitable restraints. Kelley Adamson, president and owner of A&M Forensics and Engineering, Inc., a company that deploys traffic accident reconstruction specialists, told the Express-B次元官网网址 that even if they were all correctly strapped in, such seatbelts are not always effective at speeds over 35 mph (56 kph).

Investigators have not released information on the speeds of either vehicle, but the fronts of the church bus and the pickup were heavily damaged in the wreck, and the bus was backed up onto a guardrail, surrounded by shattered glass.

"In a head-on collision like this, it's basically equivalent to the vehicle hitting a brick wall," Adamson said. "At impact speeds above that level, you're still going to suffer serious injuries, if not death."

He said older people are particularly vulnerable in high-impact collisions.

"We know that a lot of people in a crash over 40 miles per hour. they suffer internal injuries such as a rupture to the heart, liver, kidney, spleen. ... Especially for older people. They literally die of internal injuries and head injuries," Adamson said.

Twelve bus passengers and driver Murray William Barrett, 67, died at the scene, DPS Lt. Johnny Hernandez said. Another bus passenger died at a San Antonio hospital. The pickup driver, Jack Dillon Young, 20, of Leakey, Texas, was still in stable condition and the lone survivor from the bus remained in critical condition Thursday night, DPS said.

DPS Sgt. Conrad Hein said the small bus was a 2004 Turtle Top, though he did not know the specific model. Turtle Top's website features shuttle buses with capacities ranging from 17 to 51 passengers, which they bill as "a great alternative to the standard 15-passenger van." Safety concerns have long surrounded the 15-passenger vans, which are frequently used by churches and other groups, with advocates saying they can be difficult to control in an emergency.

The First Baptist Church of New Braunfels, Texas, said its members were returning from a three-day retreat at the Alto Frio Baptist Encampment in Leakey, about 9 miles (15 kilometres) from the crash site.

The collision was one of the deadliest in Texas in recent memory. Eight people were killed in May when a charter bus headed to a casino rolled over north of Laredo. In 2015, eight inmates and two corrections officers were killed when their prison bus skidded off a highway near Odesa, travelled down an embankment and was struck by a passing freight train.

Seventeen people died in 2008 when a charter bus crashed in North Texas near the Oklahoma border, and 23 nursing home residents being evacuated from the Houston area as Hurricane Rita approached in 2005 were killed when their charter bus caught fire near Dallas.

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Lauer reported from Dallas. Associated Press writers Jamie Stengle and David Warren also contributed from Dallas.

Claudia Lauer And Jim Vertuno, The Associated Press