AMAGA, Colombia — Relatives held a resigned vigil Thursday outside a coal mine in northwestern Colombia where dozens of miners were trapped and feared dead after an explosion that killed at least 16 workers. The fiery blast at the San Fernando mine, believed caused by a methane gas buildup, tore through an access tunnel that is some 1.2 miles (about 2 kilometers) long and drops to a depth of 500 feet (150 meters). Officials said there were 70 to 80 workers in the mine at the time. Provincial disaster coordinator John Rendon said the Wednesday night blast collapsed part of the tunnel. But Antioquia state mining secretary Nicolas Lopez said rescue workers who had been able to get 700 meters into the tunnel and had found it intact. The tunnel has cement-reinforced steel arches, he said, but high levels of methane and carbon monoxide were impeding the entry of rescue workers down to extraction areas where most of the trapped workers were believed located.