Environmental Protection Agency Administrator Gina McCarthy on Wednesday promised an internal and external investigation into how an agency contractor released 3 million gallons of polluted water into southwest Colorado's Animas River.

Speaking in Durango, Colo., McCarthy said the external investigation would offer "fresh eyes" to assuage local concerns about the EPA's mishap. The agency's response to the Aug. 5 accident at the Gold King Mine has been harshly criticized by state and federal lawmakers from both parties as too slow.

The accident comes at a politically fraught time for the agency, which just a week ago finalized its rule for limiting carbon emissions from power plants, the signature and most contentious environmental regulation of President Obama's tenure.

More congressional attacks on the Obama administration over the incident are expected from Republicans. House Natural Resources Committee Chairman Rob Bishop, R-Utah, whose committee oversees federal mine management, said his panel "will be conducting extensive oversight over the causes and the short-term and long-term effects of this serious situation."

McCarthy called the disaster "heartbreaking," but contended the Animas River's water quality has recovered, confirming a Colorado state report. Still, the agency will likely face lawsuits over the incident.

An agency contractor using heavy machinery punctured the mine, which contained water contaminated with heavy metals from previous mining. The release of water into the river turned it orange, and polluted water flowing into New Mexico and Utah.

McCarthy directed the agency's regional offices to halt investigative work at the mines unless there is "imminent risk," while another federal agency or "another external entity" conducts a review to determine what went wrong.

"Based on the outcome from that assessment, we will determine what actions may be necessary to avoid similar incidents at other sites," McCarthy said in her instruction to other regions.

House Oversight and Government Reform Committee Chairman Jason Chaffetz, R-Utah, sent a letter Wednesday to the EPA's internal watchdog alleging that the EPA "is not well-positioned to conduct a competent internal review in this case," and called for an independent probe.

Democrats also have criticized the EPA for its handling of situation.

New Mexico Democrats Sens. Tom Udall and Martin Heinrich and Rep. Ben Ray Lujan said Tuesday that they "were disappointed by the lack of communication between the agency and the state and local governments immediately after the spill occurred."

Environmental groups, meanwhile, have attempted to find fault with how abandoned mines like the Gold King Mine are handled, and say the federal law governing management of toxic waste from hard rock mines has remained largely the same since it was enacted in 1872.

"It's pretty standard in business that if you break something, you pay for it and other industries, including coal and oil, are expected clean up their own messes, but there's no equivalent program for the hard-rock mining industry to protect communities and taxpayers from disasters like this one," said National Wildlife Federation President Collin O'Mara.