"It is really a tragic and very unfortunate incident, and EPA is taking responsibility to ensure that that spill is cleaned up," McCarthy said. "I am absolutely, deeply sorry that this ever happened."

The accident comes at a sensitive time for the EPA, a frequent and favorite target of conservatives and pro-business groups. McCarthy spoke Tuesday as part of an event on the Obama Administration's Clean Power Plan, which mandates steep greenhouse gas emission cuts from U.S. power plants.

State and local officials in the areas affected by the spill have characterized EPA's initial response as too slow and too small. It took about 24 hours to first notify some downstream communities of the accident and the agency originally underestimated the volume of the spill.

The plume of pollution has since flowed at least 100 miles downstream to New Mexico, where towns and cities have been forced to close their intake valves to protect public water supplies.

Sen. Jim Inhofe, chairman of the Senate Environment and Public Works Committee, said Tuesday he is closely monitoring EPA's spill response.

"This has and will continue to lead to significant economic damage to local businesses, farmers, tribes and residents," said Inhofe, R-Okla. "I will work within the committee and with my colleagues in Congress to ensure the EPA is held accountable to this grave incident and that those impacted are provided the necessary support to move forward."

McCarthy pledged a thorough review of the EPA's role in the disaster, but said her current focus is on properly managing the response. So far, there have been no reported cases of anyone's health being harmed by the spilled heavy metals, she said.

"It takes time to review and analyze data, so I understand people's frustration, but we have our researchers and our scientists working around the clock," McCarthy said. "Our commitment is to get this right and make sure we are protecting public health."

Al Jazeera and the Associated Press