CHARLESTON, W.Va. (AP) â¿¿ An Ohio gas company involved in a drilling rig explosion that injured seven workers has been cleared to resume operations in West Virginia. The West Virginia Department of Environmental Protection began reviewing 19 permits it had issued to AB Resources PA LLC of Brecksville, Ohio, after the June 7 blast near Moundsville. The company complied with the conditions of the cease-operations order and corrected all underlying violations, DEP spokeswoman Kathy Cosco said Thursday. Union Drilling Inc. of Fort Worth, Texas, was sinking a well for AB Resources and another company, Dallas-based Chief Oil & Natural Gas, when it struck a methane pocket in an abandoned coal mine. As the crew began to remove the drill string, the methane exploded. The exact ignition source may never be determined, though some evidence points to equipment near the rig, said James Martin, chief of the Office of Oil and Gas. "It would be a disservice to the company, the industry and especially the injured workers to concentrate solely on what ignited the fire rather than the underlying causes and what measures need to be taken to prevent this from happening again," he said. AB Resources must now contact state inspectors when it encounters unexpected mine voids and modify its well casing plan as necessary, Martin said. It also must install gas-detection monitors on the drilling floor of each operation and take other measures to plan for well control at shallow depths. The state has yet to assess any penalties for the accident, he said. An investigation by the U.S. Occupational Safety and Health Administration is continuing. AB Resources is part of the growing rush to tap the vast natural gas reserves of the Marcellus shale field, a rich deposit that underlies Ohio, West Virginia, Pennsylvania and New York.