The bomb, which went off by accident after being hit by a construction worker in the town of Euskirchen, North Rhine-Westphalia last Friday, was of British origin, experts believe.
"We're quite sure now that it was a British aerial mine, " Rolf Vogelbacher from the bomb disposal unit of the Düsseldorf local authority told WDR broadcaster on Monday.
During World War II the Allies often bombed the town as a well-used transit point for German troops. Up to four large bombs are still found every year in the town, said local mayor Uwe Friedl.
Digger operators are well trained and very experienced with the dangers there - even the man who was killed on Friday when he hit the bomb by mistake at around 1.30pm on Friday afternoon.
Police believe the bomb had been very large. It was around 2.8 metres long and weighed 1.8 tons, 1.3 tons of which consisted of various explosives.
The blast broke windows up to six kilometres away, a police spokesman told Spiegel news site and it could reportedly be heard in Bonn - almost 30 kilometres away.
The vehicle burst into flames with the driver still inside, killing him. Thirteen others at the building site were injured, including two seriously.
Reports suggest that the effect of the blast could be felt for kilometres around Alfred-Nobel-Straße, where it took place.
"There was a huge blast wave. In the vicinity of the accident site and surrounding streets, home windows shattered and garage doors were pushed in," a police spokesman said.
“I had a feeling in my stomach that I wouldn't wish on anyone,” manager at a nearby supermarket Gerhard Nachbauer told the Westdeutsche Allgemeine Zeitung (WAZ). “The shop was evacuated within three minutes,” he said.
Business owner Victor da Silva, whose offices are 200 metres away from the blast site, told the paper that “our glass wall exploded before our eyes and roofs and streetlamps lamps fell over.”
The shock caused him to burst into tears, da Silva told the WAZ.
The ground below many Germany cities still contains unexploded bombs dropped by Allied and Soviet forces in the World War II, but most are safely defused when found.
Police have cordoned off the area and are surveying the damage.