RALEIGH, N.C. (AP) — 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. Following the disaster, Duke officials said the company didn’t know that an underground section of the pipe was made out of metal, believing instead that it had been fully constructed of more-durable reinforced concrete.