From the letter:
We, the undersigned organizations and public stakeholders, call on you to investigate the Environmental Protection Agency for its withdrawal of legal action against Range Resources in a case of apparent water contamination from hydraulic fracturing near Fort Worth, Texas. We understand that you are currently conducting an investigation of this incident at the request of six U.S. Senators. We want to ensure that your investigation encompasses issues that have recently come to light that raise questions about EPA’s commitment to protecting the public from oil and natural gas drilling pollution.