An electrical substation fire in Puerto Rico has knocked out power to 400,000 throughout the island

A resident in San Juan captured photos of an explosion that caused a large fire at the Monacillos substation.

(CNN) -- A fire at an electrical substation in Puerto Rico has knocked out power to hundreds of thousands throughout the island, utility company LUMA Energy said Thursday.

"The fire caused major blackouts across the entire island. The situation is under assessment and work is being done to restore the system," LUMA Energy tweeted, stating that restoration efforts will continue overnight.

About 400,000 customers in Puerto Rico were without power as of late Thursday, LUMA Energy said, which is more than 1 in 10 residents on the island.

The fire broke out in a transformer at LUMA's Monacillos substation, the company said. Personnel from the Puerto Rico Police, Fire Department and Medical Emergencies are at the scene and no injuries have been reported.

Calling it an explosion, Gov. Pedro Pierluisi said that both state and federal law enforcement authorities are investigating the cause.

"All the resources of the PR government are available to handle the emergency caused by the fire at the Monacillos substation in Río Piedras. Firefighters have already arrived on the scene to handle to the situation. As soon as [firefighters] confirm more details, we will let you know," Pierluisi said on Twitter.

"The priority must be to re-establish the electrical service as soon as possible and our people can rest assured that we are making sure that this is the case. At the same time, as soon as we have the results of the investigations related to the cause of this incident, whoever is responsible for it will have to answer to the People of Puerto Rico," he said.

In a press conference with local media Thursday, LUMA Energy CEO Wayne Stensby said at the peak of the outage, 800,000 customers had been without power, according to the newspaper El Nuevo Día.

"The FBI is evaluating the situation concerning the massive black out," FBI San Juan public affairs officer Limary Cruz-Rubio told CNN Thursday night, but would not comment further on the matter.

Website hit by cyberattack earlier that day

LUMA Energy said it was the victim of an earlier Distributed Denial of Service attack, in which a site is deliberately hit by massive amounts of web traffic in an attempt to overwhelm it.

"The DDoS attack was identified and has been issuing 2 million visits per second," LUMA Energy said in a statement. "As a result, many clients may have had difficulty accessing the information related to their accounts."

There was no immediate indication of any connection between the website attack and the substation explosion.

"The attack has been reported to the relevant authorities and LUMA awaits the results of an ongoing investigation," according to the press release.

LUMA is a joint venture of Quanta Services and the Canadian energy company ATCO, which was chosen by the Puerto Rican government to take over the operation of the power grid from its previous public electric utility, PREPA. LUMA has been in charge of the power grid since June 1.

CNN's Christina Carrega contributed to this report.

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