GILLETTE, Wyo. (AP) — Nobody was hurt when two oil tanks caught fire and exploded northwest of Gillette.

Fire officials are still investigating the cause.

One partially full tank and one empty tank exploded between 5 p.m. and 6 p.m. Thursday. The blast shook buildings and a plume of black smoke rose as onlookers gathered nearby.

Fire department Capt. Chris Beltz says the tank exploded when a pumper was moving oil from a separator into the tank. A separator removes water and other substances from oil before it is stored.

Beltz says lightning or welding can cause oil tank explosions but neither was the case here.

The Gillette News-Record reports firefighters contained the fire by about 7 p.m. but oil continued to burn for several hours.

Damage was estimated at $20,000 and $25,000.