A German World War II bomb caused an accident on a fishing boat off the English county of Norfolk in December 2020, injuring several people.
As the British Marine Accident Investigation Branch announced on Thursday, the fishermen's nets were "most likely" caught on the dud.
The bomb may have been dragged along the seabed and caught on an obstacle, such as a boulder.
The explosive device detonated on the seabed.
The resulting shock wave was so strong that it caused the fishing boat to sway violently.
Five of the seven crew members were seriously injured.
The hull and engine of the fishing boat were also badly damaged.
The crew was taken off the lifeboat of a nearby ship and later taken to the hospital.
Experts have now determined that it was a 250-kilogram German-made bomb from World War II.