Transcript for Thousands in Puerto Rico are told to evacuate as a new disaster unfolds

We begin with the urgent headline from Puerto Rico at this hour. A dam at imminent risk of failure, a life flaet threatening situation. A mass evacuation under way. Some 70,000 people across the island. Catastrophic flooding and mudlieds already. The cover saying it's time to get people out. We begin with ABC's rob Marciano on the scene to get tens of thousands to higher ground. Reporter: A large dam is beginning to fail. Two towns and tens of thousands of people in danger downstream. A flash flood emergency in effect right now. Officials urging people to get to high ground. This is a life-threatening situation happening now. Tonight the new emergency in this unfolding disaster. The battering of hurricane MARIA and her 40 inches of rain causing this major dam's failure. Flash flooding fersing 70,000 people to evacuate immediately. Officials saying a crack is forming in the guajataca dam. People's lives are in danger. Utter devastation being felt throughout the U.S. Territory. The chaos and desperation etched into caregiver MARIA Ortez's voice. We can't let them die. We can't let them die. We need all the help we can get. Please. Reporter: We first met her Thursday as she struggled to keep 11 nursing home patients alive. Their generator low on fuel. I see when that runs out -- What we going to do? Reporter: Those patients in sweltering conditions inside. Some clinging to life. When we returned today help had not arrived. The generator out of fuel. It's been 20 hours since we called the governor's office on her behalf. Calling again today. We had trouble getting through and we're not alone. Communications nearly impossible across the island. Drivers lined up here as far as the eye can see to access one of the only working cell phone towers. MARIA hasn't seen or heard from her mother since before the hurricane hit. Some relief knowing her parents are okay. Then the son of one of her patient's shows up with a diesel that the care center desperately needs. How long does a tank last? Eight hours. Reporter: Another temporary physician. Fix. Elation with knowing there's enough fuel to at least get through the night. Rob Marciano is back with us. We're glad to see the nursing home at least got some fuel. The other major concern you reported on the dam, tens of thousands told to get to higher ground immediately? Reporter: Yeah. The governor cut his press conference short to head out there. The military vehicles trying to get people out right now. It's going to be a dangerous night ahead David. The meantime the possible paths for hurricane MARIA as it moves towards the east coast. They seem to shift night to night rob. Reporter: We didn't give the U.S. The all clear and now they're shifting closer to home. Right now it's a cat 3 and will weaken father north by Wednesday. North Carolina, the forecast getting close to the U.S. And our spaghetti models spread out. There could be some direct impacts from MARIA in the U.S. By the end of next week. David. Rob Marciano on the ground in Puerto Rico again tonight.

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