Duke Energy, the giant utility whose spill of toxic waste into a North Carolina river last month is under federal investigation, released wastewater last week from a second site near Raleigh that state regulators said could be illegal.

Aerial photographs of two Duke coal ash ponds at the head of the Cape Fear River show portable pumps and hoses that appear to be siphoning water into a canal leading to the river.

A spokesman for the state’s Department of Environment and Natural Resources said on Saturday that its inspectors noticed the pumping while on a site visit last week. “We are investigating the utility’s actions,” the spokesman, Drew Elliot, said in an email. “While routine maintenance is allowed under the permit, discharge of untreated wastewater could be a violation.”

A spokesman for Duke, based in Charlotte, said the pumping was intended to lower the water level in the ponds, which contain a slurry of coal ash with toxic heavy metals, as part of a “routine maintenance” program and was allowed under the site’s antipollution permit.