Police have said they received phone calls about a pick-up truck driving erratically shortly before a collision between a truck and a church minibus in south-west Texas that killed 13 people returning from a retreat.
One man called the dispatch line to report that a white Dodge pick-up was swerving on the road, Uvalde Police Lieutenant Daniel Rodriguez said.
"(The caller) was scared (the pick-up driver) was going to cause an accident and asked us to send deputies," Mr 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 US 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, Mr Johnson said.
"Unfortunately, he struck a motor vehicle before anyone could respond," he said.
The Texas Department of Public Safety refused to speculate about what caused the head-on collision near the town of Concan, although one spokesman said the truck driver appeared to have crossed the centre line.
The National Transportation Safety Board sent investigators to the scene outside Garner State Park, about 75 miles west of San Antonio.
The fronts of both vehicles were heavily damaged in the collision and the bus was backed up on to a guardrail, with glass and debris scattered around.
Twelve bus passengers and driver Murray William Barrett, 67, died at the scene, DPS Lieutenant Johnny Hernandez said. Another bus passenger died later at a San Antonio hospital.
The pick-up driver, Jack Dillon Young, 20, of Leakey, Texas, was in a stable condition and the lone survivor from the bus remained in hospital in critical condition late on Wednesday.
Brad McLean, senior pastor of First Baptist Church of New Braunfels, Texas, said: "These are individuals we've sat next to and had dinner with and laughed with and cried with and worshipped with. They were part of our church family.
"I think it's the everyday interaction and relationship that has been built that, boy, those are the things that really will affect us a week from now, a month from now, a year from now."
Ten of the people killed were from New Braunfels, according to DPS, and they ranged in age from 61 to 87. They were part of a larger group of 65 people who attended the church retreat.
DPS Sergeant Orlando Moreno said: "For reasons unknown, the truck veered into the southbound lane and struck the bus head-on."
He said the wreck occurred along a curve in the road where the speed limit is 65mph.
The San Antonio-area church said on its website that the members were returning from a three-day retreat at the Alto Frio Baptist Encampment in Leakey, about nine miles from the crash site.