Text messaging is being blamed for the accident.
A federal safety official says the 19-year-old pickup truck driver involved in the crash near Gray Summit, Missouri sent or received 11 texts in the 11 minutes immediately before the accident.
The National Transportation Safety Board chairman says it's clear the pickup driver was manually, cognitively and visually distracted.
Investigators say the driver sent six texts and received five texts just before his pickup crashed into the back of a tractor truck, beginning a chain collision.
The pickup was rear-ended by a school bus, which in turn was rammed by a second school bus.
The pickup driver and a 15-year-old student on one of the school buses were killed.
38 other people were injured in the accident near Gray Summit in August 2010.
As a result of the investigation into this crash, the NTSB Tuesday again recommends banning the use of portable electronic devices by all drivers unless there is an emergency.
Read the NTSB's summary of the crash, causes and recommendations here.