Police remain unclear on how Michael Heckhoff (50) and Peter Paul Michalski (48) were able to overpower a guard and gatekeeper to make their escape, but know that they forced two taxi drivers to take them to Cologne's city centre early in the morning. There they fled on foot, after which police lost all traces of their whereabouts, they said.
Police have launched a large-scale manhunt with helicopters and sealed off parts of the city with increased security at train stations and airports.
On Friday afternoon, police said that a witness had reported a "very concrete" sighting of the two escapees outside a hardware store in Dierdorf, a city some 80 kilometres away in the neighbouring state of Rhineland-Palatinate. But checkpoints at various interregional transportation junctions have proved to be fruitless so far, they said.
Anyone who sees the two is advised to keep their distance and notify police immediately.
The Aachen men's prison from which the men escaped is considered to be among the most modern in Europe. Some 800 of the Germany's most dangerous criminals are currently serving their sentences there.