No one is confirming yet exactly what happened that knocked out power to thousands of residents and tourists on two North Carolina barrier islands.

Pile driving operations for the new Bonner Bridge on North Carolina's Outer Banks appeared to have struck a buried electric transmission line Thursday afternoon, knocking out power to thousands of residents and tourists on two barrier islands.

Exactly what happened isn't clear. The transmission line had been relocated in preparation for consruction. Equipment operators reportedly drove a steel casing into the line while constructing supports on land at the south end of the new $246 million 2.8-mile span across Oregon Inlet.

That plunged the tourism-dependent communities on Hatteras and Ocracoke Islands into darkness. Because Ocracoke is accessible only by ferry, Gov. Roy Cooper (D) declared a state of emergency and ordered visitors to leave the island by 5 pm Friday. NCDOT is stepping up its ferry service to handle the extra volume of traffic. No such order has been issued for Hatteras Island, which has a highway connection to the upper Outer Banks and the North Carolina mainland.

PCL Civil Constructors, the bridge project's prime contractor, said in a statement that it is investigating with NCDOT and Cape Hatteras Electric to "determine the exact cause of the power outage south of the Bonner Bridge" and "doing everything we can to assist in restoring power." PCL is excavating the area around the damaged transmission line to determine the extent of the damage, and hopes to issue a timeline for repairs.

Local officials have said that repairs could take anywhere from a few days to several weeks. An extended outage could have a major effect on the local economy, which is driven primarily by visitors to the remote islands' scenic beaches. Local officials estimate that 10,000 tourists were on Ocracoke when the evacuation order was issued.

Mobile Generators Dispatched

Meanwhile, several 2MW mobile generators have been dispatched to the islands to provide temporary, limited power for residents and businesses until full service can be restored. Construction of the Bonner Bridge began last year following a four-year legal battle between the state and environmental groups concerned about long-term protection of federally-protected areas on Hatteras Island.

An out-of-court settlement allowed the replacement of the inlet's existing 54-year old span to go forward in exchange for addressing several erosion-prone areas on State Route 12, the island's only highway. According to NCDOT, the new bridge will require more than 670 concrete pilings, ranging in length from 110 to 130 feet. Pilings in the area where the transmission line breach occurred are were to be hollow 54-in dia columns. The project is scheduled for completion in 2018.