PRAGUE (AP) — 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.

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.

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.