An islandwide blackout hit Puerto Rico on Wednesday, knocking out power throughout the island seven months after Hurricane Mariadestroyed its electrical grid.

Geraldo Quiñones López, a spokesman for the Puerto Rico Electric Power Authority, or PREPA, told NBC News early Wednesday afternoon that there was “zero percent” power generation on the island.

“The breakdown affected the entire electrical system and generating plants,” he said.

Crews were investigating the blackout and it could take 24 to 36 hours to fully restore power, he added.

Justo González, executive sub-director of PREPA, said during a news conference Wednesday afternoon that the blackout was caused by “a circumstance with a contractor.”

He said that the company was working to remove a collapsed tower on Wednesday when a bulldozer approached a power line, causing it to fail.

There was “a sudden, strong change of frequency and voltage,” he said.

PREPA said in a post on Twitter Wednesday afternoon that energy was restored to clients in the municipalities of Maunabo, Naguabo, Mayagüez, Arecibo, and Toa Baja, while the islands of Vieques y Culebra never lost electrical power.

The company said in an earlier post that it was prioritizing re-establishing power to hospitals, the Luis Muñoz Marín International Airport, pumping systems for the Puerto Rico Aqueducts and Sewers Authority and central banks.

The airport said on Twitter that it was running on a generator and so far no flights had been cancelled or delayed.

It’s the first time since Category 4 storm Maria hit the island on Sept. 20 that Puerto Rico has experienced a full islandwide blackout.

Some 40,000 power customers are still without even normal electricity service as a result of the hurricane.

Just last week, 840,000 customers were left without power after two electrical lines failed, causing a massive blackout. One of the lines was affected by a tree that fell during unrelated power restoration efforts elsewhere on the island.

San Juan Mayor Carmen Yulín Cruz tweeted Wednesday “The entire electrical system in Puerto Rico collapses AGAIN! Back to September 20th.”

The entire electrical system in Puerto Rico collapses AGAIN! Back to September 20th. @DavidBegnaud @leylasantiago @maddow @stephencolbertr