Russian Deputy Defense Minister Gen. Dmitry Bulgakov said on Tuesday that a string of spectacular explosions at the Kamenka ammunition depot in Siberia’s Krasnoyarsk region was probably the result of “human error,” but added that “investigative authorities and prosecutors will tell for sure.”

It is easy to see why residents of the nearby city of Achinsk found the explosions at the arms depot on Monday troubling, as they unleashed an apocalyptic fireball and vast plumes of smoke that dominated the horizon.

The blast was so powerful that it sent visible shockwaves rippling through the overhanging clouds.

According to the Defense Ministry, a fire broke out in a part of the depot where gunpowder charges for some 55,000 artillery shells are stored, causing some of the ordnance to detonate. One source within the Russian military said a single older shell blew up inside a truck and started the fire that triggered the immense subsequent detonations.

Gen. Bulgakov’s promise of an investigation tacitly acknowledged that while wildfires have been seen in the region, the nearest one was miles away from the depot, so the explosion was not likely to have been caused by embers from a natural fire.

The BBC reported that thousands of civilians were evacuated from a 12-mile area around the depot as flying debris damaged some buildings, including a school and a kindergarten. Work at a major aluminum plant near Achinsk was temporarily suspended as the staff was evacuated.

The latest updates from Russian media say a dozen people were injured, with conflicting reports that one Russian soldier may have been killed.

The firefighting effort included ten huge cargo planes and five helicopters dropping water bombs across the area. Air traffic over the site of the explosions was suspended for safety and to accommodate the firefighting effort, which ran throughout the night. A crew of 70 bomb disposal experts was dispatched to the site to secure the remaining munitions.

The BBC reported on Tuesday that shrapnel plus intact artillery shells blown through the sky by the explosions have been found in a nearby village.