(Reuters) – Puerto Rico faces a lengthy and complicated power restoration process after Hurricane Maria last week left the island’s 3.4 million residents without electricity, according to a group of top U.S. electric company executives.

The CEO-led Electricity Subsector Coordinating Council (ESCC) said in a release late on Tuesday that damage assessments must be completed to determine what human and equipment resources will be needed to restore power.

Maria struck Puerto Rico as a Category 4 hurricane last Wednesday, with winds up to 155 miles (249 km) per hour, leveling structures across the island and killing at least 16 people.

The ESCC is coordinating the restoration efforts between the U.S. government and the electric power sector.

Its members include the American Public Power Association (APPA), which represents companies like the Puerto Rico Electric Power Authority (PREPA), which serves all 1.5 million electrical customers on Puerto Rico.

“We thank (U.S.) President (Donald) Trump for authorizing 100-percent cost sharing by the federal government for 180 days of emergency work to help begin the process of repairing damaged energy infrastructure,” said APPA President and Chief Executive Sue Kelly.

The ESCC said the APPA and the electric power industry will work with government partners to secure the transportation, fuel, housing, food, water, and security that will be needed to support restoration crews once the assessments are done.

“Responding to major events like Hurricane Maria requires significant coordination among the public and private sectors,” said Tom Fanning, an ESCC co-chair.

“Puerto Rico is facing complicated logistical challenges for life and safety that need to be stabilized before full power restoration efforts can get underway,” said Fanning, who is also the chairman, president and chief executive of Atlanta-based Southern Co.

The rates PREPA charged were not enough for the utility to maintain its infrastructure, in part due to ineffective collection efforts and long-standing mismanagement that had left it in a $9-billion hole before declaring bankruptcy in July this year.

PREPA’s equipment was already “degraded and unsafe,” according to a draft fiscal report the company filed in April.