Engineer Steve McEvoy told ABC11 that inspectors found overlapping sheets of vinyl used at the base of the dam had been breached. The cause of the failure is still under investigation.
Water flowing under the dam caused sinkholes which emptied the entire 88-acre lake located south of Fort Bragg and Fayetteville.
The Hope Mills Lake dam was just replaced in 2008. About $14 million was spent building the dam after storm damage over Memorial Day weekend in 2003 damaged the old one - causing it to burst.
The town spent five years coming up with the money to replace it.
Roughly $9 million in state and federal funding was used, but the Town of Hope Mills still has a $4 million outstanding loan to pay.