Cologne city authorities conducted a controlled detonation of a large World War II bomb produced in the US on Friday evening, after failing in initial attempts to defuse the 1-ton bomb. 

"The ordinance removal service detonated the ten-hundredweight World War II bomb at 17:57," the City of Cologne said online.

"Thanks to all helpers who contributed to the smooth work."

An image of a fairly large crater, apparently the site of the detonation, was attached.

The city had also warned shortly before the detonation that even if everything went as planned, "a muffled bang will be discernible to the public, even beyond the 500-meter (roughly 550-yard) at-risk area."

After warning that they had not been able to defuse the bomb safely, a delay followed as trucks then brought tons of sand to the site to prepare for the controlled detonation.

The bomb was unearthed two days earlier during excavations at a construction site for a new medical facility in the eastern district of Merheim.

Cologne is rather practiced at defusing unexploded ordinance from the 1940s and quickly initiated plans to render the bomb harmless.

Three hospitals in at-risk radius, police go door-to-door to clear area

The city described it as its most complex bomb disposal operation since 1945, probably in large part because of the danger zone's location close to three different medical facilities including the large Cologne-Merheim Hospital. 

Three clinics with almost 650 patients had to be cleared ready for the operation.

A small number of intensive care patients who could not be relocated safely were instead moved to what was deemed a "safe room" in one of the hospitals, well enough fortified to not be at risk even if the operation did not go to plan. 

Roughly 6,400 residents were asked to leave their houses from mid-morning on Friday until the all-clear was given. A nearby school was set up as a gathering point if people had no good options of their own.

Police and other officials went door-to-door on Friday morning to check that locals had followed their instructions. 

Roughly 1,300 volunteers also assisted in various capacities.

One bus service was rerouted and another was canceled during the operation.

Regular occurrence in Cologne, one of the earliest major targets in Germany

Such operations are very common in Cologne, even as the 80th anniversary of the end of World War II approaches.

The most recent find of another, smaller bomb in the city was barely a week ago, on October 1. Another was defused on August 21. A larger 2-ton bomb was defused roughly a week before that.

The city was a crucial industrial, cultural, and modal point for Nazi Germany during the war. It was also one of the more viable targets for bombing raids launched from the British mainland while Germany occupied France, Belgium and the Netherlands before the D-Day landings of 1944.

Located in western Germany, and nearer the north than the south, it was one of the larger cities further inland — discounting the most accessible northern ports like Hamburg — that were in range of early World War II bombers and that involved crews taking somewhat fewer risks flying over anti-aircraft defenses.

Cologne was the target of the first of the "thousand bomber raids" conducted by Britain's Royal Airforce in the summer of 1942, on the night of May 30 and 31. The bombardment famously flattened a large portion of Cologne's city center, while missing its renowned cathedral on the banks of the Rhine almost entirely. 

msh/wmr (AFP, dpa)