An engineering firm has pinpointed — sort of — the cause of a 9-inch hole in a sewage pipe that allowed 18.3 million gallons of sewage to escape in late October and early November.

“Forensic engineers have concluded that the underlying cause of the failure … was likely a blow to the outside of the pipe many years ago,” said Nancy Munnikhuysen, a spokeswoman for the Hampton Roads Sanitation District, which owns the pipe.

The hole was discovered Oct. 29 during Hurricane Sandy, which likely exacerbated the old wound in the pipe, according to Munnikhuysen. The flow of sewage was not stopped until Nov. 8, and Wilroy Road was shut down for a month to make repairs.

The firm, Lewis Engineering and Consulting, determined the 30-inch pipe was dented by a heavy blow at least 10 years ago, according to the press release. The damage could date to when the pipe was installed in 1977, Munnikhuysen said.

“The investigation was something like the detective work seen in TV crime dramas, except it takes much longer in real life,” Munnikhuysen said.

The engineers visited the site, inspected the portion of pipe that broke and examined adjacent sections, she said. They collected soil and water samples to determine how quickly the environment would corrode the pipe. Laboratory tests were performed on samples of the pipe’s exterior and interior coatings, as well as the wire embedded in the pipe.

The damage went undetected for so many years because the pipe is buried beneath the bottom of the tidal marsh, Munnikhuysen said. It carries wastewater to the Nansemond Treatment Plant, more than 12 miles away.

The volume of wastewater would have increased beyond normal levels during the storm, elevating the pressure above would the weakened spot could handle, she said.

“There are no records of external damage to the pipe either during construction or at any time since it was placed in service nearly 36 years ago, leaving the actual cause of the external damage an unsolved mystery,” Munnikhuysen said.