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(CNN) —  

Power was gradually returning to Puerto Rico Thursday night after the US commonwealth was hit with a massive outage – nearly seven months after Hurricane Maria destroyed much of the island’s infrastructure and its electrical grid.

The Puerto Rico Electric Power Authority, known as PREPA, said a tree fell on a major power line, knocking out service to 870,000 customers, about half its clients. The utility tweeted an image it said was of the tree that fell across a power line near the southeast mountain town of Cayey.

Puerto Rico's Electric Power Authority tweeted an image it says was of the tree that fell across a power line near the southeast mountain town of Cayey.
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Puerto Rico's Electric Power Authority tweeted an image it says was of the tree that fell across a power line near the southeast mountain town of Cayey.

In a statement Thursday evening, PREPA executive Justo Gonzalez Torres said a major transmission line was damaged while contractors were clearing vegetation. A worker suffered burns on a hand and foot when the machine he was operating touched the tree that came down on the power line, said Jesus Martinez, director of Emergency Management for Cayey. The worker was in stable condition.

Power had been restored to about 779,000 customers by early evening, and full restoration was expected in the next few hours, PREPA said.

Major blackouts were reported earlier from the northern coastal town of Manatí to Yabucoa, roughly 50 miles southeast.

The blackout also included the capital of San Juan, the most populated area of the island.

“We are devastated at this point,” Cynthia García Coll, an educational psychologist who lives near San Juan, told CNN. “Every time this happens we go into PTSD mode.”

San Juan Mayor Carmen Yulín Cruz said earlier that it could take eight to 16 hours for electricity to return.

Cruz posted images on Twitter of police officers directing traffic on the streets of San Juan.

The mayor, shortly before 5 p.m., reported that power was returning to parts of the capital.

Hurricane Maria made landfall in Puerto Rico, home to more than 3 million US citizens, in late September.

Puerto Rico has lost 3.4 billion customer-hours of electricity service due to Maria, according to an analysis released Thursday by the economic data analytics and policy firm Rhodium Group. That made it the largest blackout in US history and the second largest in the world – after the outage caused when Typhoon Haiyan tore the Philippines in 2013, killing more than 6,000 people.

“More customer-hours have been lost in Puerto Rico due to Hurricane Maria than in the rest of the US over the past five years due to all causes combined,” according to the group’s analysis.

“We are becoming a Third World country,” García Coll said. “And we the people in the street … are working 24/7 to contribute to this country’s recovery without reliable infrastructure.”

The power authority reported last month that more than 100,000 people were still in the dark.

The US Army Corps of Engineers said more than $200 million worth of materials were to arrive in Puerto Rico in March to help the corps’ power restoration efforts. The materials included more than 7,000 poles and nearly 400 miles of conductor wire, according to Col. Jason Kirk, commander of the Corps’ district that includes the island.

The Corps of Engineers has said back-to-back 2017 disasters, the remoteness of the island and the fact that some supplies had to be manufactured for installation in Puerto Rico, slowed down work on the electric system.

The Puerto Rico Electric Power Authority faced widespread criticism late last year for signing a $300 million contract to restore power with Whitefish Energy Holdings, a small Montana-based firm with only two employees at the time of the contract.

The public utility company canceled the contract amid public outcry and its executive director stepped down in November.

San Juan resident Domingo Marqués said on Thursday that many residents have lost confidence in both PREPA and the island’s government.

“At this point, we are so helpless and hopeless,” Marqués said. “We still have so many people exposed to suffering, displacement and trauma that two hours without power is like a reminder that the problem is nowhere near to be fixed. We have just learned to live with it.”

CNN’s Spencer Feingold contributed to this report.