Records show engineers working for Duke Energy warned the company as early as 1986 that a stormwater pipe running under a North Carolina coal ash dump was made of corrugated metal and needed to be monitored for leaks.
That pipe collapsed in February, triggering a massive spill that coated 70 miles of the Dan River in toxic gray sludge. Following the disaster, Duke officials said the company didn't know the pipe was made out of metal, believing instead that it had been constructed of more-durable reinforced concrete.
The 28-year-old engineering report was among documents subpoenaed last week from the North Carolina Utilities Commission by federal prosecutors as part of an ongoing criminal investigation into the spill.
Duke spokesman Dave Scanzoni declined to comment.