Most -- if not all -- of the occupants in the Texas bus crash were wearing seat belts: NTSB

— -- Most -- if not all -- of the occupants of a bus transporting churchgoers from a retreat in Texas were wearing their seat belts at the time of the crash, according to the NTSB.

While the driver and front-seat passenger were equipped with standard three-point seat belts, the rear passenger seats were equipped with lap-only belts, said NTSB Investigator-in-Charge Jennifer Morrison at a press conference Friday.

Only one passenger on the bus -- 64-year-old Rose Mary Harris -- survived, according to authorities. Both Harris and the driver of a pickup truck that crashed into the crash, 20-year-old Jack Dillon Young, sustained serious injuries, the NTSB said on Friday afternoon.

NTSB investigators are looking to interview both Harris and Young when they are well enough to speak with authorities, Morrison said.

The bus driver and remaining 12 passengers died.

Emergency calls made to the Real County Sheriff's Office and obtained by ABC San Antonio affiliate KSAT reveal that a witness called to report a white Dodge truck, the same model involved in the crash, that was "all over the road."

"Somebody needs to stop this guy," the witness told the dispatcher. "... Somebody needs to get this guy off the road."

The NTSB completed its first full day of investigations of Wednesday's crash today, said Morrison. The NTSB's investigation is focusing on transportation safety and what could have been done to prevent future crashes and fatalities.

Morrison did not discuss the cause of the crash, adding that the on-scene investigation will continue for about a week.

The bus was carrying members of the First Baptist Church in New Braunfels, who were returning home from a three-day retreat at the Alto Frio Baptist Encampment, according to the church.

Young, who was driving a white Dodge Ram pickup truck, veered into the opposite lane of U.S. Route 83 near Concon before he crashed into the bus head-on, authorities said.

Most of the passengers on the bus were in their 80s, according to officials. The youngest victim was 61-year-old Rhonda Barlow Allen, the Department of Public Safety said.

ABC News' Emily Shapiro contributed to this report.