ISABELA, P.R. — The police came to this coastal city, nicknamed the Garden of the Northwest, in trucks, jeeps and medical evacuation vans. The city’s public safety director wore a life jacket.

Their message was simple: Get out.

“They said the dam is going to explode today or tomorrow,” Jobani Cuevas, 18, said. “For them to move us, I guess it’s pretty serious.”

The dam would not burst, officials assured residents, but the danger of flooding was real. Several hundred people who live in low-lying areas along the Guajataca Dam, about 60 miles west of San Juan, abandoned their homes Friday and Saturday, days after Hurricane Maria devastated the island. Cracks in the dam, officials said, had put surrounding areas in peril.

The island has already been dealing with a blackout after the storm knocked out its power grid. It now faces serious infrastructure problems that could inundate towns and leave tens of thousands of people without drinking water. On Wednesday, several people drowned in Toa Baja, where dam gates had been opened in anticipation of the hurricane.