Federal safety investigators issued a subpoena to Pennsylvania’s public utility regulator on Monday for documents related to a fatal explosion at a chocolate factory, escalating a months-long legal dispute over the state agency’s authority to share the sensitive information.

The National Transportation Safety Board said the Pennsylvania Public Utility Commission has refused to provide unredacted inspection and investigation reports for UGI Utilities Inc., the natural gas utility at the center of the probe into the March 24 blast at the R.M. Palmer Co. plant in West Reading.

The powerful natural gas explosion leveled one building, heavily damaged another and killed seven people. Investigators have previously said they are looking at a pair of gas leaks as a possible cause of or contributor to the blast.

The interagency dispute over five years’ worth of UGI records involved a conflict between state and federal law.

The Public Utility Commission said it could not provide the records in the format that the safety agency demanded, citing a state law that protects “confidential security information” about key utility infrastructure from public disclosure, even to other government agencies.

The commission said it offered safety investigators a chance to inspect the reports at its Harrisburg office or to sign a nondisclosure agreement, but the federal agency refused.

“This is a unique situation where a federal agency is demanding that the PUC violate state law,” PUC spokesperson Nils Hagen-Frederiksen said in a written statement. “It is unfortunate that the NTSB has rejected possible solutions to this issue, but we continue working to resolve this impasse.”

The safety board said federal regulations entitled it to the utility company records and asserted the PUC was required to turn them over.

Because federal law preempts state law, NTSB chair Jennifer L. Homendy wrote to the state utility commission chair, the PUC “has no legal basis to withhold the ... inspection reports from the NTSB in any manner.”

In addition to issuing the subpoena, the safety agency said it also barred the Public Utility Commission from having any further role in the federal probe.

“The actions of PA PUC have evidenced a lack of cooperation and adherence to our party processes and prevent your continued participation in the investigation,” Homendy wrote.

About 70 Palmer production workers and 35 office staff were working in two adjacent buildings at the time of the blast. Employees in both buildings told federal investigators they could smell gas before the explosion. Workers at the plant have accused Palmer of ignoring warnings of a natural gas leak, saying the plant, in a small town 60 miles (96 kilometers) northwest of Philadelphia, should have been evacuated.