City fire chief Stephan Neuhoff told a news conference: "The chances of pulling someone alive from the rubble are around zero."
Cologne's head municipal administrator Guido Kahlen confirmed there were still two people missing, apparently trapped in a building next to the city archives, which collapsed suddenly Tuesday afternoon.
"We are relieved that the number of people missing at the moment has not increased.... there are still two people missing," Kahlen told reporters.
Efforts to save the pair are being hampered as rescue workers have been unable to stabilise surrounding buildings, despite pouring in 300 cubic metres (392 cubic yards) of concrete overnight in a bid to secure the area. More than 200 rescue workers were still on the scene with sniffer dogs.
Officials also fear that over 1,000 years of city history could be lost, including items left to the city by figures like composer Jacques Offenbach and Nobel-Prize winning author Heinrich Böll.
There are 30 kilometres (20 miles) of shelving containing the archives' treasures, Georg Quander, head of the cultural affairs department in Cologne said.
The archives are the not just the biggest collection in Germany but "the most significant north of the Alps," he said.
The biggest danger to the archives is rainwater, Quander said. Workers at the scene were hastily erecting tarpaulins in a bid to protect the documents. Quander added it was too early to assess the extent of the damage to the cultural treasure trove.
Officials hailed the "extraordinary co-operation" between emergency services that enabled the building to be evacuated within three minutes, preventing further casualties.
"I escaped from the second floor with my girlfriend by running along a corridor that was starting to split in two," Heiko Wegener, a resident who lived next door to the archive building told public television station NTV. "I then managed to free the people on the first floor."
Emergency services also managed to evacuate some 70 residents at a retirement home as well as children from a neighbouring school.