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Hurricane Maria was strengthening fast into a monster storm Monday as it barreled toward Puerto Rico and other Irma-battered Caribbean islands.

Maria grew — in less than 24 hours — from a tropical storm into a Category 4 hurricane that the National Hurricane Center called an "extremely dangerous" system. At 5 p.m. ET, it was just 45 miles east-southeast of Dominica, an island of 72,000 people in the Lesser Antilles, and producing maximum sustained winds of 135 mph.

Maria could begin threatening the Virgin Islands on Tuesday evening and Puerto Rico by Wednesday morning, said the hurricane center, which issued hurricane warnings for Puerto Rico and its satellite islands of Culebra and Vieques.

Puerto Rico has not been hit by a Category 4 or 5 hurricane since 1928, NBC News meteorologist Bill Karins said.

Maria, however, could be "catastrophic" for Puerto Rico, which was largely spared by Hurricane Irma, Karins said. It passed 50 miles north of the island and caused only wave damage, but even that was enough to knock out power to about 1 million people.

"There's an excellent chance that Maria will be a major hurricane very close to Puerto Rico in 48 hours," he said, adding that it could also hit the Irma-devastated U.S. and British Virgin Islands.

"Maria is expected to produce total rain accumulations of 6 to 12 inches with isolated maximum amounts of 20 inches across the central and southern Leeward Islands, including Puerto Rico and the U.S. and British Virgin Islands, through Wednesday night," the hurricane center warned.

Puerto Rico Gov. Ricardo Rosselló warned Sunday the storm could bring more rain, wind and water than Irma, which killed three people there.

Rosselló said 46,000 people — or about 85 percent of customers in the metropolitan area of the capital, San Juan — remained without electricity. Another 6,000 were still without drinking water.

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Help was already on the way. A ship from the Federal Emergency Management Agency was expected to arrive early Tuesday with more than 1 million gallons of water and 111 generators, and the island was ready to house 67,000 people across 450 shelters, Rosselló said.

"The priority is to be prepared and save lives," he said.

In Condado, just east of historic old San Juan, Sonia Yanguas was appealing for help to the Almighty.

"We're praying to God that it will weaken out at sea," said Yanguas, 76, who lives in a ninth-floor apartment by the sea. "I’ve already made the essential purchases, especially water. I have my equipment, I have a radio and batteries, all the things that are so important to making sure you don’t end up without a way to communicate."

But she's not sticking around for Maria. She's heading inland to the city of Miramar.

"I’m going to prepare my apartment right now and then I’m going to a hotel," said Yanguas. "I’m going with my two sons and four grandchildren."

Ivelisse Rodriguez, who lives just south of San Juan in Guaynabo, said people were really frightened, with many lining up at gas stations trying to purchase fuel. "There's a collective hysteria," said Rodriguez, 42.

After Irma, she said, "we were without power for six or seven days and without water for nine or 10."

"Being without water was the worse than being without power," she said. "There’s thing’s you can do if you lose power, but there’s nothing that can substitute water.”

Irma killed four people and did "apocalyptic" damage in the U.S. Virgin Islands. Will Tuttle, who flew down from Hyannis Port, Massachusetts to the island of St. John help his mother rebuild after Irma, said they're boarding everything up again for Maria.

"We covered the windows with plywood and are now putting up more plywood to reinforce them," Tuttle said. "We’ll throw everything from outside into the pool — lawn chairs, chaises. It sinks, it’s safe, it doesn’t go anywhere."

On St. Thomas, Omari Williams said everybody is heeding the call to get out of their houses.

"People here are trying to get safe," he said. "Officials are encouraging more people to come to public shelters especially if their home is already compromised."

Farther south, on St. Croix, Lisa Mackay said they're ready for Maria and all they can do now is wait.

"There wasn't much we had to do for Maria because we did everything we had to do for Irma," said MacKay, who is from Memphis but married to a man whose family has living on the island for generations. "We are seriously concerned about the storm. Our homes are well built, mine included. We have hurricane shutters we put up, but the reality is it’s an unknown."

In the British Virgin Islands, where Irma killed four, Gov. Augustus Jaspert warned Maria could dump six to 20 inches of rain. He warned Maria's powerful winds could turn anything not battened down into projectiles.

Severin Pradel, who lives on Anguilla, said they;re sick of storms.

"People are frustrated and saying, 'Oh my god, I can’t believe it, we can’t take another one,'" he said. "If Maria comes then we’ll get beat up again and will have to start all over."