Updated 4:19 p.m.

A federal appeals court on Friday tossed out another key permit for the $7.5 billion Atlantic Coast Pipeline that deals with the project's effects on threatened or endangered species. It's another legal setback for the 600-mile project, but the pipeline's builders say they hope to be able to "clear up errors" with the permit, and resume construction.

A three-judge panel of the 4th U.S. Circuit Court of Appeals wrote that the U.S. Fish and Wildlife Service had made decisions that were "arbitrary and capricious" in its authorization for the pipeline.

"In fast-tracking its decisions, the agency appears to have lost sight of its mandate under the (Endangered Species Act): 'to protect and conserve endangered and threatened species and their habitats,'" the court wrote.

Dominion Energy, Duke Energy and The Southern Co. are building the project, which is designed to carry natural gas from West Virginia into Virginia and North Carolina. It's years behind schedule, and its total cost has increased by about $2 billion.

Construction began last year, but the project has been on hold since December after previous court rulings.

ENDANGERED SPECIES

The appeal decided Friday dealt with four species: the rusty patched bumble bee; the clubshell, a mussel; the Indiana bat; and the Madison Cave isopod, a crustacean.

Last year, the court vacated the Fish and Wildlife Service's incidental take statement, which usually authorizes a project to harm or kill no more than a limited number of threatened or endangered species.

Soon after, the agency revised its work and issued a new one, which environmental groups challenged.

The court wrote that it could not ignore that it took the agency "a mere 19 days" to issue the new incidental take statement and related biological opinion after the court's first decision.

"In its rush to help this pipeline company, the agency failed to protect species on the brink of extinction - its most important duty. This pipeline would blast through some of the last populations of these rare animals," said Patrick Hunter, an attorney with the Southern Environmental Law Center, which represented the environmental groups that sued.

A spokeswoman for the Fish and Wildlife Service said the agency is reviewing the decision.

DOMINION DEFENDS THE PROJECT

Lead developer Dominion Energy said in a statement Friday afternoon defended the project and said it expects to clear up problems with the permits and resume construction.

"Based on the clear direction provided by the court in today’s opinion, we expect FERC and the Fish and Wildlife Service will be able to immediately begin working to correct the errors identified by the court.

Once the new Biological Opinion and Incidental Take Statement are issued, we will seek the necessary approval from the Federal Energy Regulatory Commission to resume construction. We’re confident we remain on track to complete the project by late 2021.

The Atlantic Coast Pipeline has been the most thoroughly reviewed infrastructure project in the history of our region. We’ve taken extraordinary care to protect sensitive species and will continue doing so as we work with the agency to complete the additional analysis required by the court."

Dominion says the pipeline will lower energy costs and boost economic development, both through its construction and by increasing the availability of natural gas.

Opponents say the project will cause environmental harm and question the need for a massive natural gas pipeline at a time when they say climate change makes it imperative to invest in renewable energy.

Meanwhile, the companies are fighting another legal decision that said the pipeline cannot cross the Appalachian Trail. In late June, they filed an appeal with the U.S. Supreme Court, which still must decide whether to hear the case.