Humphreys County, Tennessee - Investigators have lost hope of finding any survivors among those missing after a catastrophic blast at an explosives factory in Tennessee, authorities said.

Humphreys County Sheriff Chris Davis told reporters on Saturday that the families of the 16 people believed to have died had been notified.

Of the 19 people initially reported missing immediately after the blast on Friday morning, three were later found elsewhere and had not been on the plant grounds near the town of McEwen at the time of the explosion.

Investigators now believe that all 16 people who were on site when the explosion occurred have died.

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DNA tests were being carried out to identify human remains, though officials said the force of the explosion and the extent of the debris field would make identification difficult.

It is unclear what caused the accident and the investigation is expected to take several days.

The company, Accurate Energetic, makes a range of explosive products, including for military purposes.

The FBI is also involved in the investigation.

Davis earlier described the situation as "hell on us. It is hell on everyone involved," in comments to CNN.

The hill near the town of McEwen where the factory stood before the incident was marked by the wreckage of vehicles and smoking debris.

Residents told broadcaster NBC that even houses in the nearby town of Lobelville were shaken by the explosion.