SAN JUAN, Puerto Rico (CBS NEWS) -- Hurricane Maria's destruction has set Puerto Rico back decades, even as authorities worked to assess the extent of the damage, Puerto Rico's nonvoting representative in the U.S. Congress said Sunday.

"The devastation in Puerto Rico has set us back nearly 20 to 30 years," said Puerto Rico Resident Commissioner Jenniffer Gonzalez. "I can't deny that the Puerto Rico of now is different from that of a week ago. The destruction of properties, of flattened structures, of families without homes, of debris everywhere. The island's greenery is gone."

Engineers on Sunday planned to inspect the roughly 90-year-old Guajataca Dam, which holds back a reservoir covering about 2 square miles in northwest Puerto Rico. The government said it suffered a large crack after Maria dumped 15 inches of rain on the surrounding mountains and that it "will collapse at any minute." Nearby residents had been evacuated, but began returning to their homes Saturday after a spillway eased pressure on the dam.

Puerto Rico's National Guard diverted an oil tanker that broke free and threatened to crash into the southeast coast, said Gov. Ricardo Rossello, and officials still had not had communication with nine of 78 municipalities.

"This is a major disaster," he said. "We've had extensive damage. This is going to take some time."

Sept. 25, 5:40 a.m.: Visitors told to leave N.C. island

Hyde County, North Carolina, officials ordered all visitors on popular vacation destination Ocracoke Island to leave on Monday, "due to the threat of hurricane Maria."

"Direct impacts from the storm include tropical storm force winds and storm surge of 2-4 feet along the Outer Banks," the Hyde County administration said in a statement released late Sunday evening. It said, due to the official state of emergency, all ferry reservations were cancelled and tolls waived, so people fleeing Ocracoke would have to line up to be evacuated on a first-come, first-served basis.

"Wind speeds could cause the suspension of ferry services early Tuesday morning, potentially making Monday night the last runs available. Decisive action is necessary for Ocracoke visitors to ensure you arrive at your destination safely," the statement said.

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