An Environmental Protection Agency contractor works on the clean up in the aftermath of the blowout at the Gold King mine, which triggered a major spill of toxic wastewater, outside Silverton, Colo., Wednesday, Aug. 12, 2015.

(AP Photo/Brennan Linsley)

The Environmental Protection Agency's inspector general is expanding its investigation into a toxic spill from an inactive Colorado mine to include questions about whether the agency was following its own rules when it triggered the blowout.

The inspector general's office said Thursday that it will add more than a dozen lines of inquiry, some requested by Congress and others raised by a review of the spill by the Interior Department.

An EPA-led contractor inadvertently released 3 million gallons of wastewater tainted with heavy metals from the Gold King Mine on Aug. 5 during cleanup operations. Rivers in Colorado, New Mexico and Utah were contaminated.

The inspector general will also look into what kind of legal protection EPA gave the contractor and whether that affected the way the work was done.