Records subpoenaed by federal prosecutors show that engineers working for Duke Energy warned the company nearly 30 years before a large coal ash spill that a storm water pipe under an ash dump was made of corrugated metal and needed to be monitored for leaks. That pipe collapsed in February, setting off a spill that coated 70 miles of the Dan River in toxic sludge. After the disaster, Duke said it did not know that section of pipe was made of metal, believing that it was made of more durable reinforced concrete. A Duke spokesman declined to comment on Thursday about the documents or whether the company carried out the recommended monitoring. The report was among the documents subpoenaed from the North Carolina Utilities Commission by federal prosecutors in a criminal investigation into the spill.