A majority of the 2,300 residents of Casselton, N.D., heeded a warning to leave town Monday after an explosion that produced huge clouds of flames and smoke, according to the Cass County Sheriff's Office. "We asked for voluntarily evacuations, and roughly 65 percent of the people left on their own," Deputy Joe Crawford said in phone interview at 3 a.m. Tuesday. Other officials said that estimate was too high, but they declined to offer an alternative number. Crawford said the evacuations were "precautionary" until the situation could be evaluated for health hazards. The explosion happened shortly after 2 p.m. Monday after a BNSF grain train derailed and crashed into a crude oil train near Casselton, which is 20 miles west of Fargo, causing tank cars to explode in towering mushroom-cloud flames. No one was injured in the crash. A monitoring crew from Little Rock, Ark., endured subzero temperatures overnight in order to evaluate the air quality and other possible hazards as a result of the accident. As they worked, the temperature dipped to 15 below with the windchill at 32 below. "It's not the kind of weather they're used to in Arkansas," Crawford said. "But we warned them and they're suited up properly." Law enforcement and emergency crews are also wearing masks as a safety measure until more is known about any threats to public health. "Right now, we're in limbo," the deputy said. The National Transportation Safety Board has a team of investigators en route to the scene, and a news conference was scheduled for 9 a.m. to update the situation. In the initial hours after the explosion, authorities told residents to stay indoors to avoid the smoke as shifting winds raised concerns among officials. Later, when residents were urged to evacuate, some drove to Fargo, where a shelter had been set up for them. BNSF spokeswoman Amy McBeth said the train carrying grain derailed first, then knocked several cars of the oil train off adjoining tracks. BNSF said both trains had more than 100 cars each. BNSF said it believed about 20 cars caught fire after its oil train left the tracks. The sheriff's office said it thought 10 cars were on fire. "It was black smoke and then there were probably four explosions in the next hour to hour and a half," said Eva Fercho, a Casselton resident who saw the fiery aftermath. The cars were still burning as darkness fell, and authorities said they would be allowed to burn out. An estimated 11 to 12 crude oil unit trains depart daily from the oil region in western North Dakota. Lacking sufficient pipelines, 69 percent of the state's oil is currently shipped to market by rail. The main railroads, BNSF and Canadian Pacific, have tracks through the Twin Cities. Fercho said the BNSF main line runs right through Casselton, and just two blocks from her home. "We are very thankful it didn't happen in the city limits," she said. The accident also spared the Tharaldson Ethanol plant, west of Casselton. Plant Manager Ryan Carter said the accident was about 2 miles away from the plant, but the burning tank cars were visible from there. "It was pretty much flames and smoke," said Carter, who estimated that about 30 tank cars were involved. Carter Hackmann, who lives about a mile away from the site of the wreck, said he heard at least three explosions, and took photographs from his house of billowing flames and smoke that resembled a mushroom cloud. Ryan Toop, who lives about a half-mile away, said he heard explosions and drove as close as about two city blocks to the fire, which erupted on a day when temperatures were below zero. "I rolled down the window, and you could literally keep your hands warm," Toop said. Jeff Zent, a spokesman for Gov. Jack Dalrymple, said the National Guard was on alert if needed. North Dakota officials have said that even more crude oil is expected to move by rail in 2014. The state is approaching 1 million barrels per day in output, and trails only Texas and the separately counted Gulf of Mexico in U.S. oil production. In July, a runaway train loaded with North Dakota crude oil crashed in Lac-Mégantic, Quebec, killing 47 people and destroying that city's downtown as tank cars exploded and burned. The Associated Press contributed to this story.