The National Transportation Safety Board (NTSB) is trying to determine the cause of the crash Wednesday on Highway 83 in Texas that left 13 people dead – all but one of them senior citizens.

According to news reports, a pickup truck driven by 20-year-old Jack Dillon Young of Leakey, Texas, veered from the northbound lane into the southbound lane and struck a church van carrying people home from a three-day retreat.

Area 911 dispatchers reportedly received at least three calls prior to the accident from people reporting a pickup truck driver operating his vehicle in an “erratic” manner while traveling north on Highway 83.

The Texas Department of Public Safety said all of the victims were in the van and ranged in age from 61 to 87. The driver of the van was also killed. One passenger, 64-year-old Rose Mary Harris, survived.

Young survived and was airlifted to a San Antonio hospital to be treated for his injuries.

Texas Gov. Greg Abbott expressed his condolences in a statement on Twitter, saying, "We are saddened by the loss of life and our hearts go out to all those affected."