Environmental Protection Agency Administrator Gina McCarthy said her agency is "accepting full responsibility" for a contractor that accidentally spilled millions of gallons of polluted water last week into western Colorado's Animas River, where cleanup efforts are ongoing as federal officials seek to maintain contamination.

The Aug. 5 spill occurred when an agency contractor operating heavy machinery punctured a barrier holding back water containing mining tailings at the Gold King Mine near Silverton, Colo. Three million gallons of water polluted with heavy metals poured into the river and turned it orange, and the tainted water spread into New Mexico and Utah.

Republicans and local residents have attacked the agency for what they called a slow response to the spill. McCarthy, who said that she expects lawsuits, said the agency is "absolutely deeply sorry" and is using the "full depth and breadth of the agency" to address the "tragic" event.

The EPA announced that McCarthy will travel to Durango, Colo., and Farmington, N.M., on Wednesday, a move that comes after Democrats and Republicans from the Four Corners region called on her to see the damage up close.

"I understand people's frustration, but we have our researchers and our scientists working around the clock," she said at a Washington event hosted by think tank Resources for the Future. She added that there have been "no reported cases of anyone's health being compromised" and that water pollution levels are on a "downward trajectory to pre-event conditions."

GOP lawmakers on Capitol Hill want to know more about how EPA handled the situation. House Science, Space and Technology Committee Chairman Lamar Smith, R-Texas, wrote to McCarthy on Monday requesting a briefing on the incident.

"It is concerning that the agency charged with ensuring the nation's waters are clean is reportedly responsible for the toxic water spill at Gold King Mine. A spill of this magnitude could be devastating for the families who live nearby and depend on the Animas River in their daily lives," Smith wrote.

Colorado's top health official said Tuesday that preliminary tests showed the spill "doesn't appear" to pose health risks, according to news reports. But the river, which is a drinking water source and recreation hotspot, remains closed as the EPA and local officials conduct more tests.

"Clearly we have added responsibility here. But we're working with the state of Colorado, we're working with the state of New Mexico, we're working with the local officials in a joint command center. They are part of the decision-making here and we want to keep it that way," McCarthy told reporters after the event.

"Right now we made sure that the downstream intakes for public water supplies were protected. And we're also working with folks that have private drinking wells along there to actually work with each one of them to provide bottled water," McCarthy added.