Residents of Casselton, N.D., returned to their homes Tuesday afternoon after being assured that a fire from a nearby train derailment was under control and the air was safe. Their return followed 24 hours of tension and fear that played out amid subzero temperatures.

At one point, about 65 percent of the town's 2,400 residents voluntarily left their homes as strong winds blew potentially hazardous, acrid smoke from a massive fireball toward town.

The incident renewed debate about how to safely transport massive amounts of crude from oil-rich North Dakota.

Casselton resident Eva Fercho spent the night at her daughter's home 25 miles away in Moorhead, Minn. The family dog also made the uneasy trip. Fercho returned home at midmorning after hearing state officials assure residents that the air was safe. Late Tuesday, smoke still wafted nearby, and soot was crystallized on snow and trees.

The Burlington Northern Santa Fe tracks run straight through the middle of Casselton, 20 miles west of Fargo. The Fercho family lives a few blocks from the tracks.

"It's always been scary to us knowing that any kind of train crash is a possibility," she said. "But with all the crude that is coming through, that raises the risk, and that was proved with this incident. I hope some safeguards are put into place."

National Transportation Safety Board representatives remained on site Tuesday afternoon. At a late-afternoon briefing, an agency spokesman said the fire on the locomotives was so severe that it probably destroyed the data recorders in the oil train's two lead locomotives. Investigators hope to interview crew members on Thursday.

Meantime, BNSF hazardous-materials crews were working to clear the wreckage and extinguish smoldering fires. The Cass County Sheriff's Office warned that small fires or smoke clouds might flare up as damaged cars are moved.

The wreck happened when a westbound train carrying soybeans headed to Minot, N.D., derailed west of Casselton just after 2 p.m. Monday. An eastbound train carrying highly volatile petroleum crude bound for Missouri hit the derailed train, causing the fire. Two locomotives of the eastbound train were destroyed and 20 cars derailed, the NTSB said. Despite the collision and fire, no injuries were reported.

The train carrying crude included 106 cars, BNSF said. About half of the cars detached from the wreck and the rest were involved in the accident.

One rail tank car can hold about 700 barrels of oil, according to the Energy Information Administration, the Energy Department's statistical arm.

Residents said the blasts went on for hours after the derailment, shaking their homes and businesses.

NTSB board member Robert Sumwalt said the 112-car grain train had originated in Royal, Neb., and was bound for the state of Washington. The oil train originated in Fryburg, N.D., and was bound for Hayti, Mo. The oil train included three locomotives and 103 tank cars. Two tank cars were full of sand to act as buffers between the product cars and the locomotives at the front and rear of the train.

The grain train crew started its day at 12:15 p.m. in Dilworth, Minn., just under two hours before the wreck. The oil train crew started work at 6:10 a.m. in Mandan, N.D., and had been on duty about eight hours.

Videos were sent to 'the cloud'

Each locomotive contained data recorders and forward-looking video cameras. Although two of the data recorders probably were destroyed, the others likely will provide useful data, Sumwalt said. The videos upload almost continuously and investigators already have examined the last 20 seconds showing the collision sequence, he said.

The NTSB has requested data from a "hotbox detector" stationed 4 miles east of the accident to look for hot wheels and axles, as well as any dragging equipment on the grain train. All of the data will be correlated with the videos and analyzed later, Sumwalt said.

"What we do know is that the grain train, there was one car that in particular had sort of gone over onto the opposing track, and it fouled that track," he said. "It was the engine of the oil train that struck this midpoint car of the grain train."

The DOT 111 cars that contained the oil are designed to carry nonpressurized loads of a variety of liquids. Federal authorities have called them inadequate for hazardous loads and many are being replaced with safer models. It appears that none of the cars involved in the accident were of the newer, reinforced design, Sumwalt said.

Casselton Mayor Ed McCon­nell estimated that dozens of people could have been killed if the derailments had happened within city limits. He said it is time to "have a conversation" with federal lawmakers about the dangers of transporting oil by rail.

"There have been numerous derailments in this area," he told the Associated Press. "It's almost gotten to the point that it looks like not if we're going to have an accident, it's when. We dodged a bullet by having it out of town, but this is too close for comfort."

North Dakota is the No. 2 oil-producing state, trailing only Texas, and a growing amount of that is being shipped by rail. The state's top oil regulator said earlier this month that he expected as much as 90 percent of North Dakota's oil would be carried by train in 2014, up from the current 60 percent.

The number of crude oil carloads hauled by U.S. railroads surged from 10,840 in 2009 to a projected 400,000 in 2013. Despite the increase, the rate of accidents has stayed relatively steady. Railroads say 99.997 percent of hazardous materials shipments reach destinations safely.

A train carrying crude from North Dakota's Bakken oil patch crashed in Quebec last summer and 47 people died in the ensuing fire.

Shipping oil by pipeline has to be a safer option, McConnell said Tuesday.

The railroad has set up a claims center for residents and businesses affected by the derailment and evacuation.