SAN JUAN, Puerto Rico — Puerto Rican officials rushed to evacuate tens of thousands of people downstream of a failing dam and said they could not reach more than half the towns in the U.S. territory as the massive scale of the disaster wrought by Hurricane Maria started to become clear on Friday.Government spokesman Carlos Bermudez said that officials had no communication with 40 of the 78 municipalities on the island more than two days after the Category 5 storm crossed the island, toppling power lines and cell phone towers and sending floodwaters cascading through city streets.Officials said 1,360 of the island's 1,600 cell-phone towers had been downed, and 85 percent of above-ground and underground phone and internet cables were knocked out. With roads blocked and phones dead, officials said, the situation may be worse than they know."We haven't seen the extent of the damage," Gov. Ricardo Rossello told reporters in the capital.More than 15 inches of rain fell on the mountains surrounding the Guajataca Dam in northwest Puerto Rico after Maria left the island Wednesday afternoon, swelling the reservoir behind the nearly 90-year-old dam.Authorities launched an evacuation of the 70,000 people living downstream, sending buses to move people away and sending frantic warnings on Twitter that went unseen by many in the blacked-out coastal area."This is an EXTREMELY DANGEROUS SITUATION," the National Weather Service wrote.