Czech explosive experts detonated a World War II bomb on Friday at a major chemical plant in northwestern Czech Republic where it was found last week, authorities said.

Besides a few broken windows, the controlled explosion didn’t immediately seem to cause serious damage, police said in a statement. The plant near the city of Litvinov belongs to Polish oil company PKN Orlen.

WWII-ERA BOMB DISCOVERED BY COLORADO CONSTRUCTION WORKERS

The bomb was found during construction work last week on Aug. 21 away from a building. Experts decided not to transport the 250-kilogram (550-pound) bomb to a safer place to dispose of it, because it contained a chemical mechanism designed to delay the explosion, a rare discovery in the country.

They covered the bomb with hundreds of sandbags before the explosion. A nearby road was closed and trams halted operations before the explosion at about noon (1000 GMT). Police sealed off an area 2 kilometers (more than a mile) from the site of the explosion.

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The refinery, which was located on Czechoslovak territory occupied by Nazi Germany during the war, was producing fuel for Nazi troops. It was repeatedly targeted by Britain’s Royal Air Force.