The FBI is investigating how Gov. Tom Wolf’s administration issued construction permits for the $5.1 billion Mariner East project, the latest official inquiry into the contentious cross-state pipeline project that carries highly volatile gas liquids to a Delaware River export terminal.

The federal probe has been running for at least six months and started in response to similar inquiries launched in the last year by the district attorneys of Delaware and Chester Counties as well as state Attorney General Josh Shapiro, according to three sources who spoke on the condition of anonymity because they were not authorized to publicly discuss the case.

The FBI’s investigation, first reported by the Associated Press, has focused on whether Wolf administration officials forced staff of the Pennsylvania Department of Environmental Protection to ignore shortcomings and approve the pipeline’s construction permits, according to the sources, who include current and former Wolf administration officials. A fourth source described the investigation as in its preliminary stages.

The $5.1 billion project is a key link in the state’s effort to promote Pennsylvania shale-gas development, and has been strongly supported by organized labor and the energy industry. But its construction has provoked sharp opposition from residents and aroused fears about pipelines transporting highly volatile liquid fuel near homes, schools, and nursing homes.

Critics, who have been frustrated by state government’s support for the project, welcomed the federal inquiry. The project is being built by Sunoco Pipeline LP, a subsidiary of Texas-based Energy Transfer LP, one of the nation’s largest pipeline companies.

“From the very beginning, and at many times along the way, we have raised serious questions about the permitting process of the Mariner East pipeline project," State Sen. Andy Dinniman (D., Chester) said in a statement. "I hope that this development sheds a bright light on those questions and more.”

The Clean Air Council, which has mounted legal challenges to the construction permits that unearthed evidence showing the close communications between state officials and the pipeline’s owner, said the FBI inquiry reinforced its concerns.

“Clean Air Council raised concerns from the beginning that these permits were being rushed, and the consequences have been disastrous,” Joseph Otis Minott, the council’s executive director, said in a statement. He said the project has subjected Pennsylvania residents “to untold devastation to their farms, homes, and drinking water.”

The Guardian reported in April that Patrick McDonnell, the DEP secretary, was in nearly daily contact with the then-CEO of Sunoco Pipeline, Mike Hennigan, and reported on those conversations to the governor’s staff.

Wolf deputy chief of staff Yesenia Bane, who is married to a former oil and gas lobbyist, was also frequently in touch with some of her husband’s clients, the U.K. newspaper reported, based on text message and email exchanges obtained through a public records request.

The Pennsylvania Ethics Commission in April dismissed a complaint against Bane, saying it was unable to establish that she used her position to benefit her husband’s business interests. But the commission, in its letter to Bane, noted that her activity created a perception “that your action in fulfilling the governor’s policy agenda was a way for you to financially benefit your husband and/or his employees.”

Wolf’s spokesperson J.J. Abbott on Tuesday referred to previous statements that the communications between administration officials and Sunoco’s agents showed nothing more than coordination of information and schedules routine for a project of the pipeline’s size.

State environmental regulators have hit Sunoco Pipeline with $13.5 million in fines since 2017 for violations related to construction of the Mariner East pipeline system.

Sunoco Pipeline is building three adjacent pipelines to transport natural gas liquids such as propane across state from the Marcellus Shale region to a terminal on a former oil refinery site in Marcus Hook.

Energy Transfer said the company had not been contacted by the FBI about the Mariner East.

A spokesperson for the Philadelphia office of the FBI, which oversees agents in Harrisburg, said she could neither confirm nor deny the existence of an investigation. The U.S. Attorney’s Office for the Middle District of Pennsylvania declined to comment.

Path of the Mariner East 2 pipeline in Pennsylvania.

Chester County District Attorney Thomas P. Hogan, who launched the first criminal probe of the pipeline’s construction last year, said Tuesday he has three prosecutors forging ahead. He welcomed news of the FBI’s involvement.

“It doesn’t impact our investigation, which has been going full steam ahead,” he said. “However, it’s always good to see federal authorities bringing their resources to the table in an investigation of this complexity and magnitude.”

Hogan in late September also notified Sunoco that his office plans to file a civil nuisance action in 60 days unless the company corrected several problems, including exposed underground pipes and leaks, or “inadvertent returns,” of drilling fluid during the pipeline’s construction.

The project began operations in 2014 when Sunoco began carrying gas liquids in a repurposed pipeline that had been built in the 1930s to carry refined products, such as gasoline and heating oil, to western Pennsylvania.

Sunoco later expanded the project by adding two pipelines that roughly follow the same route, but construction of the new pipelines has involved extensive excavations and horizontal drilling through densely populated Chester and Delaware Counties. Sinkholes have opened along the route and construction has contaminated streams and private water wells.

The Mariner East 2 began operating in December, and the third pipeline, the Mariner East 2X, is set to be complete in 2020.

Spotlight PA reporters Angela Couloumbis and Rebecca Moss contributed to this article.