SAN JUAN, PUERTO RICO — Tens of thousands of residents in northwestern Puerto Rico were ordered to evacuate Friday amid fears that a dam holding back a large inland lake was in imminent danger of failing because of damage from Hurricane Maria’s floodwaters.

Officials worried that as many as 70,000 people could be in the path of a massive amount of rushing water in the event the Guajataca Dam releases into the Guajataca River, which flows north through low-lying coastal communities and empties into the ocean.

The dam suffered a “fissure,” Puerto Rico Gov. Ricardo Rosselló said in a news conference Friday afternoon. Residents in the municipalities of Quebradillas, Isabela and part of San Sebastian could be affected if the dam collapses, he said, and it could be a catastrophic event.

“To those citizens … who are listening: Please evacuate,” Rosselló said. Buses were sent to ferry residents out of harm’s way. “We want your life to be protected … Please, if you’re listening, the time to evacuate is now.”

The urgent situation Friday came more than 48 hours after Hurricane Maria slammed into Puerto Rico’s southeastern coast as the most powerful storm to strike the island in more than 80 years. It was a reminder that Maria’s impact on Puerto Rico is far from over; officials still have little sense of the scope of the damage the island sustained as a communications and power blackout continued to affect nearly everyone in the U.S. territory.

Authorities on Friday reported six deaths across the island. Three of the fatalities occurred in the municipality of Utuado as a result of mud slides, Puerto Rico’s public safety department said in a statement. Two others died in flooding in Toa Baja, and one other person died in Bayamón when a panel struck him in the head. More deaths are likely to be reported in coming days as search and rescue crews reach previously inaccessible areas, officials said.

“We are aware of other reports of fatalities that have transpired by unofficial means but we cannot confirm them,” said Héctor M. Pesquera, secretary of the public safety department.

Though damage assessments have been nearly impossible, early reports reveal an island ravaged by Maria’s high winds and torrential rains, with roofs peeled open like tin cans, neighborhoods waterlogged, and trees that were lush just days ago now completely stripped bare of leaves. The hurricane plowed through the entire 100-mile island, with the eye tracking diagonally from the southeast to the northwest.

Aerial view of the devastation at Palma Real Shopping Center in Humacao, a municipality on the east side of Puerto Rico. (Photo by Dennis M. Rivera Pichardo for The Washington Post)

“Every vulnerable house here made out of wood was completely or partially destroyed during the path of the eye of the hurricane,” Rossello said of an island where many homes are constructed of wood foundations and zinc roofs. “Puerto Rico has endured an horrific ordeal.”

The lack of communications has isolated rural areas of the island. Just 15 percent of the island’s communication towers are working, and some of the island’s transmission towers have collapsed. Up to 85 percent of its fiber cables are damaged.

Power remains completely out on the island, and just 25 percent of it has water service.

Shock has given way to frayed nerves as officials warned that it could be months before power is restored to some areas, and there is no indication of when communications infrastructure will be fixed. In the capital, streets were choked with traffic as people tried to find loved ones and spent hours waiting in line for gas.

The De La Cruz family could not find fuel on Thursday. On Friday morning they waited in line for six hours at one of the open stations here, and there were still 20 cars in front of them. Gabriel De La Cruz and his wife, Luisa, took turns fanning their 1-year-old son, Ismael, who sat sweating in the hot car, wearing only a diaper.

“This is all we have,” De La Cruz, a 30-year-old restaurant cook, said of the car. They lost their home and all their belongings in the storm.

Residents searching for loved ones in remote areas met downed trees, power lines and other debris. News was particularly scarce from the southern and central parts of the island, as well the tiny island of Vieques to the east.

“Even worse than not having power or water, which we’ve unfortunately become accustomed to, a communications blackout was the real anxiety-inducing feature … we haven’t really dealt with it before,” said Miguel A. Soto-Class, president of the Center for a New Economy, a San Juan-based think tank. “Are people dead and suffering or are people like we are, bruised but fine? The not knowing part is just terrible.”

Soto-Class stood on the roof of his home, the only place where he could get a cellphone signal, as Coast Guard helicopters buzzed overhead. He has not been able to get in touch with family on the island’s west coast and considered driving to find them. He abandoned the plan after realizing he does not know the condition of the roads.

Puerto Rico, with 3.5 million U.S. citizens, also is facing a crisis because of its geography: It is an island dependent on air and sea for supplies and volunteers. The immediate response that occurred after Hurricane Harvey in Houston, where volunteers from Louisiana headed in during the storm, or during Hurricane Irma in Florida, where utility trucks were pre-positioned to turn on power, is impossible here.

“It’s not like you can just drive a tractor-trailer,” said Melissa Mark-Viverito, the Puerto Rican-born president of the New York City Council. “That adds a whole other layer of logistical challenge to it.”

New York Gov. Andrew M. Cuomo and U.S. Rep. Nydia M. Velázquez (D-N.Y.) flew here Friday, bringing 34,000 bottles of water and nearly 10,000 Meals Ready to Eat.

The enormity of what they had just been through — and what was yet to come — appeared to be sinking in for many people, including those who considered themselves hurricane-hardened.

“This storm was something,” said Geraldo Ramirez, 36, a resident of San Juan’s La Perla neighborhood. “I was here for Hurricane Georges back in ’98, and that was hard to believe, how badly it affected the island. But this, Maria, was something altogether different.”

Ramirez lives in a small three-story purple house near the waterfront on Calle San Miguel with his sister, her husband and their two children. His house, a sturdy cinder-block structure, was built 17 years ago and did not suffer much structural damage. But rain and ocean water managed to find its way into every room.

Asked when the power would likely return to his small neighborhood, he answered, without hesitating, “Three or four months, at least. Maybe six.”

“But it’s okay, we will make do,” he said. “We are used to it and it’s always the same. Georges, Hugo, we lose power and we lose water. But we know how to survive.”

[‘I just need my mother’: The frantic quest to contact loved ones in Puerto Rico]

Leaning against the wall of his carport in his light blue one-story home in coastal Loiza, Jorge Diaz, 72, had only one thing on his mind: his brothers and his sister, and how one day soon he would be with them in Orlando.

“There’s only one thing I’m waiting for,” he said. “The airport to open.”

One block down and across the street, Lizmarie Bultron, 39, trudged through calf-high water to exit her home, about a block away from the beach.

“Everything I had is gone. I lost my whole house, the only thing left is the floor,” Bultron said. She looked at her feet, still ankle-deep in water. “And this, this water won’t be gone for at least a month. All we can do is wait. Wait for help to come. That’s the only choice. But no one has come yet. Not FEMA, not anyone.”