Tom Hogan, Chester County's district attorney, sent the company that owns the Mariner East pipelines a five-page letter demanding it hand over and preserve a list of documents and electronic records.
In a statement, he said he will demand that "every aspect of these pipelines be conducted safely, or we will bring into play all of the tools of the criminal justice system."
Hogan also suggested that Gov. Tom Wolf and state utility regulators had not done their jobs.
"Quite frankly, I thought the governor or the Public Utility Commission would step in and make sure it's being done safely, and it became apparent to us that it was not," Hogan said in an interview Wednesday.
Hogan also accused the company of treating Chester County residents poorly, frightening people who had complained with "subtle and not-so-subtle bullying."
Hogan is targeting Sunoco Pipeline's Mariner East 1, 2 and 2X projects stretching across southern Pennsylvania. The projects have weathered more than $13 million in fines and at least two temporary shutdown orders from state agencies, while residents living along the pipelines are suing the parent company, Energy Transfer LP of Dallas, Texas, in federal court.
Energy Transfer said in a statement Wednesday it is confident that it hasn't violated criminal laws and called the allegations "baseless."
It also said it has worked closely with state officials and inspectors to respond to citizen concerns and that safety "is our first priority and this project was planned and implemented based on that fact."
Hogan said a company lawyer contacted his office Wednesday to see if they could sit down for a discussion.
The pipeline construction is blamed for sinkholes that developed within 50 feet (15 meters) of homes and near an Amtrak rail line in southeastern Pennsylvania, drawing a temporary shutdown order.
At another point, the state Department of Environmental Protection accused Sunoco Pipeline of a "lack of ability or intention" to comply with environmental regulations after citing it for dozens of violations involving polluting waterways.
Environmental advocacy groups tried unsuccessfully to halt the construction, saying it would unleash massive and irreparable damage to the state's environment and residents.
Hogan said he is investigating potential crimes, including causing or risking a catastrophe, criminal mischief, environmental violations and corrupt organizations by everyone from pipeline workers to corporate officers.
The utility commission said Wednesday that it has a number of active investigations and cases involving Sunoco and the Mariner pipelines, including complaints brought by its pipeline safety inspectors and private citizens.
Wolf's office said the Department of Environmental Protection and the utility commission have provided "unprecedented oversight" of the Mariner East projects, enforcing "stringent and aggressive" permit conditions to protect the environment.
"The commonwealth is living up to our promise to hold this project accountable to the strong protections in the permits and our abilities under existing law," the governor's office said.
The roughly 300-mile (480 kilometer) Mariner East 1 was originally built in the 1930s to transport gasoline westward from a refinery in Marcus Hook, near Philadelphia. It was renovated to carry natural gas liquids such as propane and ethane eastward to the refinery from booming natural gas production fields above the Utica Shale and Marcellus Shale formations.
The 350-mile-long (563 kilometer) Mariner East 2 and 2x are new, wider pipelines designed for the same purpose. They are under construction along the same route, but stretch farther, through West Virginia's northern panhandle and into Ohio.
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