Volunteer groups are organizing to provide the community with updates and wrangle private relief flights, and a handful of nonprofit groups are on the ground here. The Humane Society of the United States normally focuses on animal care, but when its workers arrived, the situation was so dire they started organizing humanitarian flights to bring in diapers, water, baby food and other supplies for people.

Some residents have already fled the island, worried about spending months without power and about how they would get medical care if the island’s only hospital runs out of diesel for its generator.

Tammy Simpson sat in line at a pharmacy in Isabel Segunda, waiting for an insulin prescription. Her doctor left the island because of ill health, she said, but for the moment she would not follow him.

Ms. Simpson said she had run out of cash, and there were no open banks or working teller machines on the island where she could get more. So how could she pay for anything? Even if she made it to San Juan, where would she stay, when every hotel was full? Where would she escape to when every outbound flight was packed? At least here in Vieques, she and her daughter had a supply of rice, tomato sauce and canned foods.

“I can’t really go anywhere,” she said.

Marco Calzada sat nervously nearby, waiting to get more insulin for his diabetic father, who was down to his last dose.