Bernhardt said it’s crucial the impacts be thoroughly studied. “For offshore wind to thrive on the outer continental shelf, the federal government has to dot their I’s and cross their T’s,” he said.

Avangrid sank as much as 0.9% to $50.34. It was trading at $50.37 shortly before the close in New York.

The project south of Martha’s Vineyard is crucial to the future of offshore wind in the U.S. It’s the first of several massive wind farms planned off the East Coast. Massachusetts, New York, Maine and New Jersey are all counting on turbines at sea to achieve aggressive clean energy targets. Their efforts are expected to spur an estimated $70 billion offshore wind industry in the U.S. over the next decade.

Vineyard Wind spokesman Scott Farmelant called the Interior Department’s decision “a surprise and disappointment.”

“We urge the federal government to complete the review as quickly as possible,” he said.

400,000 homes

Last month, Avangrid Chief Executive Officer Jim Torgerson said on a call with analysts that the project “will be challenged to move forward in its current configuration” if the final study is not issued by early September.

Vineyard Wind, which would have enough capacity to supply about 400,000 homes, has a contract to sell power to three Massachusetts utilities for $65 per megawatt-hour — a lower-than-expected price that encouraged other states to map out plans for offshore wind. That cost, however, hinged on a lucrative tax credit. The company may not be eligible to claim it if construction is delayed too long.

An Interior Department review explored how Vineyard Wind may affect other industries and resources, including marine life. But the National Marine Fisheries Service raised concerns it looked too narrowly at potential cumulative effects on fishing, prompting the supplemental review, Bernhardt said.

“If it’s going to be developed, it needs to be developed in a way that everyone gets to say, at least, that we didn’t shave the ball,” Bernhardt said.

One Farm

Although the Interior Department has sold the rights to develop wind farms up and down the U.S. East Coast — and states have committed to buying the renewable power they generate — so far just one has been built: a small, 30-megawatt facility near Block Island, Rhode Island, that went online in 2016.

Expectations for offshore wind development are now greater than originally analyzed by the Interior Department, the Bureau of Ocean Energy Management said by email. The supplemental analysis will revise the agency’s assessment of the cumulative impacts of that development, the bureau said.

“There’s a lot riding on getting this right,” said Timothy Charters, vice president of government and political affairs for the National Ocean Industries Association. “It’s critically important that the department produces a decision that can give industry confidence that these projects can be permitted and will be sustained under U.S. courts.”

Wind developers are struggling to navigate a thicket of obstacles to installing turbines in U.S. waters, as fishermen, shippers, conservationists and the Pentagon raise concerns with the plans. Vineyard’s experience underscores that “state offshore wind deployment targets provide a clear market for project sponsors but they do nothing to resolve the development hurdles,” said Timothy Fox, a vice president at Washington-based research firm ClearView Energy Partners.