Missouri Highway Patrol officials said Friday that bus driver inattention and short following distances may have contributed to the crash on Interstate 44 that claimed the lives of two teens Thursday.

But the Highway Patrol investigation of the multivehicle crash near Gray Summit will take weeks, and the National Highway Safety Board began its own inquiry Friday, saying it may be more than a year before it can provide any answers.

Jessica Brinker, 15, of St. James, Mo., and Daniel Schatz, 19, of Sullivan, were killed in the crash. Fifty-five people — most of them, like Jessica, students on their way to an annual summer trip to Six Flags St. Louis — were injured, including two seriously.

"At this point, this early in the investigation, every possibility is on the table," Christopher Hart, vice chairman of the National Transportation Safety Board, said Friday night. "We have not taken anything off the table yet."

Earlier Friday, the Highway Patrol identified Michael D. Crabtree, of Kearneysville, W.Va., as the driver of a 2007 Volvo truck tractor, operating without a trailer, that was the first to be struck in the chain reaction crash in the eastbound lanes of the interstate.

Crabtree was stopped in traffic that had backed up from a construction zone about one mile to the east, according to the Highway Patrol.

Schatz, driving a 2007 GMC Sierra pickup, apparently didn't notice that Crabtree's truck had stopped and hit it from behind.

Then a school bus driven by Katherine P. Shackelford, 75, of St. James, hit the rear end of Schatz's truck, pushing it on top of the tractor, police said.

Shackelford told investigators she had moved to the left to avoid a vehicle stopped on the right side of the road. She said she checked her mirrors and was not able to stop in time. A Highway Patrol report said Shackelford was "inattentive." Missouri Highway Patrol Cpl. Jeff Wilson said driver inattention can involve someone taking his or her eyes off the road.

A second bus, also from the St. James School District and driven by Kelly M. McEnnis-Mullenix, 38, hit the rear of the first school bus. The impact pushed the first bus on top of the other two vehicles, police said. In its report, the Highway Patrol said McEnnis-Mullenix was following the first bus too closely.

Wilson said that determination was based on physical evidence at the scene and the final position of the vehicles.

The state said both buses were owned and operated by Copeland Bus Service LLC of St. James and were leased to the St. James School District.

Copeland officials did not return a telephone call seeking comment on Friday. Wilson said the entire Copeland bus fleet was inspected by the Highway Patrol in March. The two buses involved in this week's accident passed the inspection, he said.

Investigators for the National Transportation Safety Board said they would spend about a week here looking into the crash.

Hart, of the safety board, said the crew of 15 was here "because there are issues of long-standing interest to the NTSB," including school bus safety, highway work zone safety and the potential for crash-sensing devices on school buses and other commercial vehicles that would warn drivers or even automatically apply the brakes.

Hart declined to speculate on the cause of the crash and said the board won't publish its final report for at least 14 months.

Hart said one of the investigative groups will look into human factors, including the actions and ages of the drivers.

In a statement posted online Friday evening, St. James School District Superintendent Joy Tucker wrote: "We are working with the State Highway Patrol and our transportation contractor to determine exactly what happened and what steps can be taken to help prevent accidents like this in the future. We are going to do everything we can to make sure that something like this never happens again."

Despite the tragic toll, industry experts say and government studies show that school buses remain one of the safest means of transporting children.

On average, 19 school-age children a year die in crashes involving school buses or vehicles transporting children for school-related events, according to the National Highway Traffic Safety Administration.

Nearly three-quarters of bus-related deaths involve pedestrians being hit by the bus or vehicle.

"What happened (Thursday) is a very rare occurrence," said Keith Henry, director of transportation at the Independence, Mo., School District and board member with the National Association for Pupil Transportation. "There is nothing safer than the big yellow school bus."

Henry, formerly the executive director of support services at Mehlville School District, said school buses are built with safety as the top priority. Drivers are subject to background and driving checks, and then trained. As a result, he added, there are 600,000 school buses in the country transporting 24 million children to and from school every day with very few incidents.

High-profile bus accidents often are followed by a call for passenger restraints, he acknowledged.

While Henry said he is neutral on the subject, he said such a decision should be based on fact, not emotion, with thorough tests by the National Highway Traffic Safety Administration.

In a 2002 report to Congress, the agency found that requiring the use of lap belts on large, new buses would provide little if any benefit in reducing serious or fatal injuries in 'severe frontal crashes." But it found that lap and shoulder belts used together could provide a benefit on larger buses when used properly.

Cynthia Billhartz Gregorian and Patrick M. O'Connell of the Post-Dispatch contributed to this report.