CONCAN, Texas (AP) — Federal officials on Thursday began an investigation into a head-on collision between a pickup truck and small church bus in southwest Texas that killed 13 senior adults returning from a church retreat.

The Texas Department of Public Safety refused to speculate on the cause of the Wednesday afternoon crash outside Garner State Park, about 75 miles (120 kilometers) west of San Antonio, although one spokesman said the truck driver appeared to have crossed the center line.

The fronts of both vehicles were heavily damaged in the collision and the bus was backed up onto a guardrail, with glass and debris scattered around.

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 in stable condition and the lone survivor from the bus remained hospitalized in critical condition late Wednesday, DPS said.

"These are individuals we've sat next to and had dinner with and laughed with and cried with and worshipped with," Brad McLean, senior pastor of First Baptist Church of New Braunfels, Texas, told reporters Thursday. "They were part of our church family."

He added, "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 in the wreck 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 retreat, with most taking their own cars for the getaway.

Authorities said the vehicles collided about 12:25 p.m. on U.S. 83 near the town of Concan. At a briefing near the crash site Wednesday night, DPS Sgt. Orlando Moreno said the wreck occurred along a curve in the road where the speed limit is 65 mph.

"For reasons unknown the truck veered into the southbound lane and struck the bus head-on," Moreno said.

Hernandez was more circumspect early Thursday, saying only that the collision remains under investigation.

The National Transportation Safety Board investigation is underway, NTSB spokesman Terry Williams 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 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 9 miles (15 kilometers) from the crash site.

Church officials were "ministering to family members to help them deal with this tragedy" and counselors will be available Thursday at the church, according to the statement.

Texas Gov. Greg Abbott and his wife, Cecilia, offered their condolences.

"We are saddened by the loss of life and our hearts go out to all those affected," their statement said. "We thank the first responders working on the scene in the wake of this unimaginable tragedy, and ask that all Texans join us in offering their thoughts and prayers."

Wednesday's 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 Odessa, traveled 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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This story has been corrected to show it was DPS Sgt. Orlando Moreno, not DPS Lt. Johnny Hernandez, who said the pickup truck crossed the center line Wednesday.