A huge explosion rocked Venezuela\'s biggest oil refinery and unleashed a ferocious fire Saturday, killing at least 39 people and injuring more than 80 others in one of the deadliest disasters for the country\'s key industry. Balls of fire rose over the Amuay refinery, among the largest in the world, in video posted on the Internet by people who were nearby at the time. Government officials pledged to restart the refinery within two days and said the country has plenty of fuel on hand to meet domestic needs and export commitments. The explosion shattered walls of nearby shops, ripped out windows from homes and left the surrounding streets covered with rubble and metal scraps. President Hugo Chávez declared three days of mourning and ordered an investigation to determine the cause of the explosion. \"This affects all of us,\" he said by phone on state TV. \"It\'s very sad, very painful.\" Vice President Elias Jaua, who traveled to the area, said on state television late Saturday that the dead included 18 national guard troops and that six of the bodies had not yet been identified. Other officials said earlier that a 10-year-old boy was among the dead. The blast occurred about 1:15 a.m. when a natural gas leak created a cloud that ignited, Oil Minister Rafael Ramirez said. \"That gas generated a cloud that later exploded and has caused fires in at least two tanks of the refinery and surrounding areas,\" Ramirez said. At least 86 people were injured, nine of them seriously, Health Minister Eugenia Sader said at a hospital treating the victims. She said 77 with light injuries were released. Almost 20 hours after the explosion, flames reaching nearly 100 feet into the night air still crackled, giving off searing heat felt by the residents about 1,000 feet from the refinery. Officials said firefighters had largely controlled the fire at the refinery. A prosecutor was appointed to lead the investigation into the cause, and troops were deployed to the area.