A dynamite blast triggered the underground explosion that killed 51 miners in West Germany's worst such disaster in 25 years, the prosecutor's office in Kassel said in a preliminary report. Shortly after the June 1 disaster, officials said they believed methane gas had exploded in the Stolzenbach mine in Borken, about 70 miles northeast of Frankfurt. But prosecutor Stephan Walcher said the fatal explosion was sparked when workers set off dynamite--considered a normal procedure--to close a depleted shaft. The blast apparently ignited coal dust in the air. Medical examiners concluded that only five of the victims were killed in the blast; the others died of carbon-monoxide poisoning.