(CNN) -- Human factors and road construction issues will be the focus of a federal investigation into the highway accident involving two school buses in Missouri on Thursday.
The National Transportation Safety Board has launched a probe of "all of the circumstances surrounding the accident," board Vice Chairman Christopher A. Hart told reporters.
"We are here to determine the cause of this accident, not to determine liability, and not to determine blame," Hart said in St. Louis, Missouri.
Two people died in the crash, which occurred on Interstate 44 near Gray Summit, 40 miles southwest of St. Louis.
A 15-year-old girl on one of the buses was killed, as was Daniel Schatz, the 19-year-old son of Republican state Senate candidate Dave Schatz, according to CNN affiliates KMOV and KTVI. Dave Schatz told KTVI: "I will see him again. As Christians we have that belief, and it's all we can cling to."
The safety board said a "bobtail tractor was rear-ended by a pickup truck which was then struck in the rear by a school bus. That school bus was then struck by another school bus."
It said the accident occurred in an area where there apparently was roadway construction and congestion.
Hart said investigators expect to be on the scene initially for seven to 10 days and might return later. He said officials want to hear from "any witnesses who have not already been interviewed who either saw the accident or have factual information pertinent to the accident."
He couldn't say when the final accident report would be available, but said such reviews have typically been completed more than a year after the incidents.
Along with the two deaths, as many as 50 people were injured in the wreck. A 16-year-old girl was in serious condition at St. John's Children's Hospital in St. Louis.
Video from the scene showed one school bus resting atop the back end of a tractor-trailer and the other school bus behind it. Later video showed a crane removing one of the buses from the tangled wreck.
KTVI reported the two school buses involved in the crash were carrying high school band students from the St. James, Missouri, School District who were on their way to a Six Flags amusement park in Eureka.