SAN JUAN, Puerto Rico -- The lights are now back on for more than 1 million customers in Puerto Rico as additional utility crews and material continue to arrive on the island, according to a U.S. Army Corps of Engineers commander.The Corps, along with the Puerto Rico Electric Power Authority, other government agencies and contractors, have worked nonstop since the island was hit by two major hurricanes just two weeks apart in September. The storms devastated the entire island and left dozens of people dead, tens of thousands displaced and even more in the dark.About 70 percent of customers now have power, with plans to go above the 90 percent mark by the end of March, said Col. John Lloyd, commander of the Army's Task Force Power Restoration.Around 450,000 customers still remain without power."I can ensure you that you're going to see more and more crews on the ground and we're going to get more and more lights on," he said Tuesday during a live update over Facebook. "It is the No. 1 priority and I don't want people to lose hope."Roughly 4,000 personnel are on the ground working to get power back to normal. Another 1,000 linesmen are expected to arrive on the island by next month, according to a Corps of Engineers news release published last week.One of the focus areas for them will be the Caguas region, a hilly area south of San Juan. Rugged terrain there and elsewhere on the island, Lloyd said, has slowed progress, and made it difficult to rebuild transmission lines."I see over the next couple of months, a lot of the hardest work is still in front of us, unfortunately," he said.Power generation plants located in the south can no longer bring electricity to the more populated northern parts of the island due to the damaged lines, he said.Many residents, he said, do not realize the magnitude of storm damage seen in hard-to-reach areas, where numerous towers have been knocked down. One line that provides power to the Humacao region, for instance, had 48 towers destroyed."The entire line was on the ground and it has to be completely rebuilt," he said, adding it can take up to three days to replace just one tower. "That's going to take time to do that."About 26,000 of the 40,000 utility poles requested -- made up of concrete, wood or metal -- have now been shipped to the island and continue to be installed every day, he said.When feasibly safe to do so, workers have also salvaged components from damaged poles -- cables, insulators, transformers -- to expedite the process."Anything that we can salvage and reuse to get lights on, we do that," the colonel said.It can be an emotional event, he said, when lights are turned on in neighborhoods for the first time in months and residents cheer and honk their car horns."It's a humbling experience to see that happen," he said. "I can't tell you enough about the strength of the people of Puerto Rico for enduring without power and for the many other challenges they face."Generators are also being distributed out to communities as part of temporary power solutions."This has been the largest mission we've done for a disaster," he said. "We've installed more generators here than we have for any other disaster before this and we continue to install generators where they're needed."In other relief efforts, contractors working for the Corps of Engineers installed their 50,000th "blue roof" this month, which represents three-quarters of buildings approved under the program. The temporary roofs are intended to last 30 days and give Puerto Rican homeowners time to find a permanent fix, according to a news release.The Corps has also collected at least 2 million cubic yards of debris -- more than half of the amount it has been tasked to remove from the island.For the colonel, the biggest issue in his operations has been new material arriving to the island."The Puerto Rican [power] grid system, specifically, has to be manufactured off the island and brought here. That's still our biggest challenge," he said. "But it's getting better every day. We're seeing more containers with that material coming in and [we're] getting them out to crews."