Firefighters on Friday continue to manage a massive blaze that started Thursday and destroyed a 1 million-square-foot medical supply warehouse in Tracy, California.

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The fire at the Medline Industries facility started around 1 p.m. Thursday and prompted evacuations at the building and some surrounding warehouses, including a FedEx facility and an Amazon warehouse, NBC affiliate KCRA of Sacramento reported.

On Friday, drivers were allowed to return and load supplies at most businesses in the area.

Police have said there were no reports of injuries, KCRA reported, but the fire sent dark plumes of smoke into the air and debris into nearby neighborhoods.

South San Joaquin County Fire Authority Chief Randall Bradley said the fire was believed to have started at the roof of the Medline building, according to KCRA. When crews arrived, he said they found “heavy fire” coming from the roof.

“High winds, high temperatures, low humidities made firefighting very difficult,” he said.

Crews were unable to fight the fire from the inside, and the sprinklers were not operating, but Bradley said he was not sure why. A lack of water pressure at nearby hydrants also created issues, Bradley said, according to KCRA.

He added that the failure of the water systems “significantly impacted” the firefight, KCRA reported.

Deputy Chief Brian Bagley said Friday that the sprinklers were recently tested, according to KCRA. But without the sprinklers, he said the building was fully consumed in under an hour.

Bradley estimated that crews would be fighting the fire for days to come.

“This is such a unique fire,” Bradley said, according to KCRA. “There’s probably three or four of these in the history of our country…this size of warehouse fire.”

About 120 of the 900 warehouse employees were there when the fire started, he added.

Officials said sparks from the fire started smaller blazes nearby. One fire that spread to a FedEx warehouse facility was fully extinguished, KCRA reported.

Air quality, however, remains a concern.

Officials in San Joaquin County have said that respiratory irritants, toxic gases, carcinogens and “at least one neurotoxin” are present in the smoke and could affect nearby communities. Those nearby should remain inside with doors and windows closed.

Medline Industries says on its website that it is the “largest provider of medical-surgical products.”

In a statement shared with KCRA, the company said that all affected Medline employees had been accounted for.

NBC News has reached out to San Joaquin County fire officials and the Tracy mayor’s office for more information.