Chesapeake Energy Corp,the Oklahoma-based firm is the No. 1 driller in Ohio.
Rig Count Interactive Map by Baker Hughes, an energy services company.
Shale Sheet Fracking, a Youngstown Vindicator blog.
The Ohio Environmental Council, a statewide eco-group based in Columbus.
Earthjustice, a national eco-group.
People's Oil and Gas Collaborative-Ohio, a grass-roots group in Northeast Ohio.
Concerned Citizens of Medina County, a grass-roots group.
No Frack Ohio, a Columbus-based grass-roots group.
Fracking: Gas Drilling's Environmental Threat by ProPublica, an online journalism site.
Pipeline, blog from Pittsburgh Post-Gazette on Marcellus shale drilling.
Allegheny Front, environmental public radio for Western Pennsylvania.
The Pittsburgh Post-Gazette reported on Friday that federal health and environmental officials are investigating a drilling site in Washington County in southwest Pennsylvania.
The federal agencies are trying to determine whether Range Resources Inc.'s Yeager Marcellus shale drilling site caused toxic air and groundwater pollution that damaged the health of local residents.
Federal officials have been looking into the problem in Amwell Township since March 2011.
Three families last month filed a personal injury lawsuit against the company.
Range has denied the charges.
Among the agencies involved are the U.S. Environmental Protection and the Agency for Toxic Substances and Disease Registry, part of the federal Centers for Disease Control and Prevention.
Reporter Don Hopey said the EPA confirmed that the Amwell Township site is one of three in Pennsylvania and one of seven nationwide that are part of a congressionally mandated federal study looking on the impacts of fracking on groundwater. That study will not be completed in 2014.