Witnesses said the truck driver ran through a rail crossing Friday and slammed into the train.
The driver, a conductor, and four people on the train were killed. It's expected to take up to a year to pinpoint the cause of the crash.
Meantime, a federal investigation into the truck company revealed it had been repeatedly cited for crashes and unsafe driving.
Investigators are also looking into the truck driver's medical history, experience and training.
Autopsies on the victims were expected within the next few days, with toxicology test results on the truck driver due a few days after that.
Officials said that the flashing lights at the crossing, which are set to blink for 25 seconds before a train approaches, would have been visible from a half-mile away if a motorist was driving at the highway's 70-mph speed limit.
The Associated Press contributed to this report.