(CNN) -- Residents of a North Dakota town got the all-clear to return home after a fiery crash this week involving a derailed train and a second one carrying crude oil.
The wreck sent massive flames into the sky. Residents vacated homes amid fears of dangerous smoke.
Crews will be able to access the site near the town of Casselton on Wednesday, where they hope to collect the rest of the data needed to finish their probe, safety investigators said.
But information taken from recording devices has been revealing, said National Transportation and Safety Board spokesman Robert L. Sumwalt.
A video camera at the head of the oil train recorded the crash as it slammed into a car of a derailed grain train.
"We looked at the last 20 seconds of the forward facing video from the oil train. And basically it shows the collision sequence," Sumwalt said.
Derailment, then crash
When the oil train arrived, the other train transporting grain and soy bean had already derailed, and one of its cars was lying in the oil train's path, he said.
The oil train slammed into it and burst into flames.
All oil trains in the United States are also outfitted with event recorders, but the crash scorched those mounted in the front of the train. "We are not optimistic that we will be able to retrieve data from those two recorders," Sumwalt said.
The ones in the rear engine are intact, and NTSB officials hope to reveal its data Wednesday.
On the ground, they will search for signs of the cause of the derailment, including marks on wooden cross ties and on rails, Sumwalt said.
They will also examine data from hotbox detectors, which are located along the tracks and pick up unusual sources of heat. They want to see if the grain train had overheating wheels or axels or whether it had equipment dragging on the ground.
Early Tuesday, crews had tested the air for toxic fumes emitted when flames and pillars of smoke towered into the sky near the town.
Their all-clear meant that residents could return home Tuesday afternoon. Nearly two thirds of the town's some 2,500 residents had vacated their homes after authorities issued a voluntary evacuation order.
After the collision, the sheriff's office said that there were no injuries and everyone was accounted for.
CNN's Steve Almasy, Aaron Cooper and Carma Hassan contributed to this report.
™ & © 2014 Cable News Network, Inc., a Time Warner Company. All rights reserved.