Drivers of cars, trucks and buses evidently continued to enter the tunnel after the explosion. Soviet troops, fearing that the explosion might have been a rebel attack, closed off both ends with tanks, trapping many inside. Some burned to death; others were killed by smoke and by carbon monoxide escaping from vehicles whose drivers kept their engines idling to stay warm in the freezing cold. As many as 700 Soviet troops and 2,000 Afghan soldiers and civilians may have died.