The Environmental Protection Agency (EPA) suspended site investigations and cleanup work at 10 polluted mining complexes due to fears the conditions may lead to spills of contaminated water, the Associated Press reported.

The sites stretch across four states, with three in California, four in Colorado, two in Montana and one in Missouri.

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EPA officials are worried that work on each site could lead to a repeat of last month’s massive spill at an inactive Gold King Mine in Silverton, Colo. An EPA team working on the site triggered a 3 million gallon blowout of toxic sludge that contaminated rivers in three states, leading to health warnings.

As a result, the EPA said it is being exceedingly careful about its work on similar sites.

"We want to take extra caution before we initiate any work," EPA Assistant Administrator Mathy Stanislaus told the AP.

The agency stopped work on the 10 polluted sites last month, but hadn’t disclosed specifics until now.

In recent years, the EPA has been trying to clean up roughly 500,000 abandoned mines across the United States.

A government watchdog report delivered to Congress on Friday said that while the agency has been identifying the contaminated mine sites, it has done little to determine the impact of those sites on the people and environment.

That is why the EPA said it is being more cautious with these 10 sites. The agency wants to have an emergency plan in place in case of any accident, internal documents showed.

Officials were heavily criticized for not having an adequate response plan before the Gold King Mine blowout.