Work continued along the Atlantic Coast Pipeline’s (ACP) 600-mile route after a federal appeals court late Tuesday agreed with environmental groups challenging the project and vacated a key permit in an order that could halt construction in certain areas indefinitely.

As part of a broader consolidated case brought by the Sierra Club, Defenders of Wildlife and Virginia Wilderness Committee, the petitioners argued that the U.S. Fish and Wildlife Service’s (FWS) incidental take permit failed to set clear limits for affected species. Issued under the Endangered Species Act (ESA), the permit is required for activities that could result in the take of, or a negative impact to, threatened wildlife.

“We conclude, for the reasons to be more fully explained in a forthcoming opinion, that the limits set by the agency are so indeterminate that they undermine the incidental take statement’s enforcement and monitoring function under the Endangered Species Act,” the court said in its order vacating the permit.

ACP spokesperson Jen Kostyniuk said the decision only impacts construction activities directly covered by the permit in “certain defined areas along the route,” adding that construction would move forward as scheduled. The project backers continue to aim for a 2019 in-service date.

“We will fully comply as required while we continue to construct the project,” Kostyniuk said. “Although we disagree with the outcome of the court’s decision, and are evaluating our options, we are committed to working with the [FWS] to address the concerns raised by the court’s order.”

ClearView Energy Partners LLC noted that FERC and other agencies rely on incidental take findings during National Environmental Policy Act reviews to determine if mitigation efforts outlined in project permits comply with the ESA.

The Sierra Club applauded the court for recognizing that the FWS, “like other agencies involved in permitting the Atlantic Coast Pipeline, failed to impose meaningful limits or adequately scrutinize the project.”

The environmental groups claimed that all construction must be halted without the incidental take permit. ACP has already received several notices to proceed with construction and work is ongoing. Kostyniuk said the company as of Wednesday afternoon had not yet stopped any construction work, adding that it must first determine the areas where it has to do that. The company is currently working to identify where its actions would have an impact on the species at issue.

“Our next steps will be to consult with the U.S. Fish and Wildlife Service, who we expect will revise the incidental take statement to provide limits that are more specific,” Kostyniuk said.

An FWS spokesman told NGI that the agency is reviewing the court’s decision and determining its next steps.

While ACP does not know when the revised incidental take permit will be prepared, Kostyniuk said the company has “conducted extensive survey work” for all the affected species over the last four years, adding that there is “a robust record on which to resolve this matter in an expedited manner.”

ClearView analysts said it appeared obvious last week during oral arguments that the incidental take permit was possibly in jeopardy as the FWS legal team during oral arguments seemed to fail at persuading judges of its validity. While the petitioners fought to suspend construction, ClearView said the exact mechanism to do so wasn’t made clear. The firm said late Tuesday that the Federal Energy Regulatory Commission would likely order construction suspended in the specified areas while the FWS amends the incidental take permit.

The court also did not say when it would issue a full opinion. The case includes another challenge to a right-of-way for the project issued by the National Park Service, which has not yet been addressed.

ACP, backed by Dominion Energy Inc., Duke Energy, Piedmont Natural Gas and Southern Company Gas, would originate in West Virginia, traversing Virginia and North Carolina to move 1.5 Bcf/d to the Southeast.

The project, like other Appalachian infrastructure, has been controversial. It was approved by FERC in October in a rare split decision. Echoing concerns that ACP serves essentially the same purpose as the Mountain Valley Pipeline, Commissioner Cheryl LaFleur argued that the two pipelines should be merged given their potential environmental impacts.

ACP is also not the only project to have hit a snag recently in federal court. Late last year, the U.S. Court of Appeals for the District of Columbia Circuit halted construction of the Atlantic Sunrise pipeline project for three days to consider an emergency stay filed by a coalition of environmental groups that were challenging that project’s FERC certificate. That ordered was eventually lifted.