Granholm on Thursday visited the southern coastal town of Santa Isabel, one of several towns in the area hit by a power outage in June that left 10,000 customers in the dark.

That same month, another widespread outage left some 350,000 customers without power across Puerto Rico, prompting Gov. Pedro Pierluisi to order an investigation

“It is unacceptable,” Granholm said of the situation.

The outage in Santa Isabel and nearby towns prompted Luma, a private company that operates the transmission and distribution of power, to install emergency generators.

Luma also launched a $4-million effort to transport and install a new mega transformer in the area, but the plan failed after crews discovered that an “internal problem” rendered the equipment unusable, further enraging customers and government officials.

Luma recently said it would relocate another transformer based in the southeast coastal town of Maunabo, but the announcement angered the local mayor, who blocked access to the equipment, saying his town depended on it.

Luma has said that transformer is not essential to providing electricity to Maunabo.

On Sunday, Luma warned that at least 11 transformers in electric substations across Puerto Rico are “vulnerable,” and if they were to fail, tens of thousands of customers would be left without power.

Puerto Rico’s power grid remains frail nearly seven years after Hurricane Mariapummeled the island as a powerful Category 4 storm, leaving some without electricity for up to a year.

While the storm razed the power grid, it had long been crumbling given a lack of investment and maintenance by Puerto Rico’s Electric Power Authority, which is s till trying to restructure more than $9 billion in debt.

Prior to Maria, renewable energy generation in Puerto Rico was at 3%. It has since grown to 9% thanks to homes and businesses that have been able to afford to install rooftop solar and storage, according to the Institute for Energy Economics and Financial Analysis.

Amid the outages, Puerto Ricans have been hit with power bill increases that many have decried. Puerto Rico’s electricity rate is now 41% higher than the average U.S. rate.