RALEIGH, N.C. — Records subpoenaed by federal prosecutors show engineers working for Duke Energy warned the company nearly 30 years before a massive coal ash spill that a stormwater pipe running under an ash dump was made of corrugated metal and needed to be monitored for leaks.

That pipe at a North Carolina dump collapsed in February, triggering a spill that coated 70 miles of the Dan River in toxic gray sludge. After the collapse, Duke officials said the company didn't know an underground section of the pipe was made out of metal, believing instead the pipe had been fully constructed of more-durable reinforced concrete.

Duke spokesman Dave Scanzoni declined to comment Thursday about the documents and whether the company implemented the monitoring recommended by its engineers.

Duke, the country's largest electric utility, serves much of west-central Florida since its merger with Progress Energy in 2012.

The 28-year-old engineering report was among documents subpoenaed last week from the N.C. Utilities Commission by the U.S. Attorney in Raleigh as part of a grand jury investigation into the spill. The Associated Press filed a public records request with the state agency, which was responsible for regulating Duke's 33 coal ash pits in North Carolina up until 2010.

Starting in the mid 1970s, the commission mandated Duke to submit independent engineering studies every five years affirming the safety of the huge earthen dikes holding back millions of tons of ash and contaminated water from rivers and lakes.

Coal ash is the by-product left behind when coal is burned to generate energy. It contains numerous toxic substances, including arsenic, selenium, chromium, beryllium, thallium, mercury, cadmium and lead. Some of the chemicals in wet ash can also corrode metal.

Duke hired Law Engineering Testing Co. to perform the required inspections at the Dan River Steam Station in Eden, N.C. In its 1986 report, the Charlotte firm noted that part of the pipe was made of metal.

"Part of this culvert is constructed of corrugated metal pipe, which would be expected to have less longevity of satisfactory service than the reinforced concrete pipes," the report says.

Among the eight recommendations made at the end of the 45-page report, the engineers specifically suggest Duke check on the pipe under the coal ash dump at least every six months. The report said the inside of the pipes should be checked for leaks any time there was a significant difference between how much was flowing in and how much was flowing out.