According to investigators, the bus did not stop at the rail crossing.
"We've come to the preliminary conclusion that the bus did not stop at the crossing prior to entering it," said Ken Suydam, the National Transportation Safety Board's investigator in charge.
The bus driver also may not have been able to see the train because of a bend in the tracks.
The collision was due, in part, because there were no indicators -- retractable gates, flashing lights or bells -- to alert drivers of an oncoming train moving across the path of the road. The installation of such alert mechanisms would further prevent any similar accidents from occurring.
Jim Hall, the director of the National Transportation Safety Bureau said that no school bus should ever cross train tracks that do not have lights or gates.
Kayla Silvers, Daniel Pack, and Amber Prichett died as a result of the collision, and four other children and the driver were seriously injured as the train ripped the bus from its chassis and dragged it for 100 yards.
Investigators said that the horn on the CSX train sounded for at least eight continuous seconds before the collision. They said a radio was on inside the bus and there were several conversations taking place.
Suydam said he could not say whether bus driver Rhonda Cloer tried to stop the bus. The vehicle's brakes were being examined and Suydam said the bus's speed had not been determined.
Cloer has refused to be interviewed by investigators, referring questions to her lawyer, Suydam said.
Using a locomotive still streaked with yellow paint from the crash, the investigators re-enacted the moments before the crash, steering the freight train through the crossing, checking its braking distance and the volume of its whistle.
NTSB Chairman Jim Hall boarded a duplicate of the school bus destroyed in the crash to observe the driver's visibility at the crossing and to study its seat belts.
"The bottom line is school buses should not go over railroad crossings of this type," Hall said. "This is not a safe situation for children."
The NTSB said its final report would not be released for months.
Hall said states should have taken more seriously an NTSB report last year calling for more warning devices at railroad crossings. He promised to look into ways more crossings could be equipped with crossbars and signals.
Funeral service were scheduled Friday for Kayla Silvers, 6, and Daniel Pack, 9. Both died instantly in the crash. Weekend services are scheduled for 9-year-old Amber Pritchett, who died Wednesday night at T.C. Thompson Children's Hospital in Chattanooga, Tenn.
One of the injured children, Brittay Gaddis, 8, was released Wednesday from the children's hospital. She said she doesn't remember the impact but does remember releasing her seat belt afterward and walking out of the wreckage.
"I believe the seat belt saved her," said Anita Beavers, Brittany's mother.
Hundreds of people gathered on the lawn at the Murray County courthouse in Chatsworth for a candlelight vigil Wednesday night.
"All I can say is to cherish them. You never know how much you'll miss them till they're gone," said David Silvers, Kayla's father.
Among those present was Georgia Gov. Roy Barnes.
"I pray that Kayla and Daniel's deaths cause us to seek understanding rather than vengeance and blame that this tragedy can help bring us closer together," he said. He also said he planned to review Georgia laws governing stoplights and gates at railroad crossings.
Led by a mass choir from churches all over the county, the group sang Amazing Grace, raising their candles toward the heavens as the song neared its end.