Chester County has sued Sunoco over its controversial Mariner East pipelines, asking the court to prohibit the company from starting construction on two county-owned properties, the county commissioners said Wednesday.

This marks the first time county officials have initiated a civil suit over the project, although in February they intervened in an existing legal challenge brought by several residents from Delaware and Chester Counties against Sunoco and its parent company, Energy Transfer Partners, involving safety risks.

On Friday, Sunoco representatives told county employees that they intended to begin open trench construction of Mariner 2 on Chester County Library property in West Whiteland Township, the commissioners said. That excavation method, however, is not permitted on county land without special permission.

“Sunoco must understand that the county owns this property,” Michelle Kichline, chair of the Chester County Board of Commissioners, said in a statement, “and we have the right to ensure as they cross county land that adjacent neighbors and our citizens are not adversely affected in any way.”

The commissioners are asking Chester County Court to restrict Sunoco from constructing pipelines on the library land in Exton, as well as on nearby Chester Valley Trail property.

Sunoco’s $5.1 billion plan is to build three adjacent pipelines, which would transport natural gas liquids such as propane from western Pennsylvania to the Sunoco refinery in Marcus Hook.

Over the last several months, Chester County District Attorney Thomas P. Hogan, Pennsylvania Attorney General Josh Shapiro, and Delaware County District Attorney Katayoun M. Copeland have launched criminal investigations into the project’s construction, citing environmental risks and safety concerns.

A spokesperson for Energy Transfer Partners could not immediately be reached Wednesday.