(Reuters) - U.S. federal energy regulatory commissioners said they were “troubled” that Ohio found signs of diesel fuel in drilling fluid samples near a spill that occurred during Energy Transfer Partners’ construction of the Rover natural gas pipeline.

The April spill occurred during horizontal directional drilling under the Tuscarawas River in Ohio and released about 2 million gallons (7.6 million liters) of drilling fluid into a wetland.

In its application to build the pipeline, Rover told the U.S. Federal Energy Regulatory Commission (FERC) that its drilling fluid would be composed of a “slurry made of nontoxic/non-hazardous bentonite clay and water.”

“We are troubled by the Tuscarawas River (horizontal directional drilling) spill and the indications that diesel fuel is present in the drilling mud,” FERC’s acting Chairman Cheryl LaFleur and Commissioner Colette Honorable said in a joint statement.

ETP was not immediately available for comment.

The presence of diesel fuel in the drilling mud is inconsistent with the commitments made by Rover on which the commission relied to certificate its project, the commissioners said, noting they fully support the action of FERC staff and the FERC Office of Enforcement to investigate these issues.

On May 10, FERC staff ordered Rover to cease new horizontal drilling. Several energy traders and analysts said this drilling ban could delay the project, which is expected to enter service in two phases in July and November.