Officials suspect work on a new 3.9-km metro line as the most likely cause of the early afternoon collapse of the city’s historical archive building in Cologne, Germany, on March 3.

Buildings adjacent to the six-floor archive structure, which was erected in 1971, were demolished. By the morning of March 4, two people were thought missing, though no injuries had been reported.

A Cologne city hall spokesman says the collapse occurred over a few minutes as ground beneath the archive caved in. The buildings were some 20 meters above new twin tunnels of Cologne Transport Company’s (KVB) North-South line, due to open in two years.

However a spokesman for the metro contractor Bilfinger Berger A.G. Wiesbaden, says a definitive cause of collapse has yet to be found.

The archive building stood above the new metro just north of the line’s Severinstrasse station.Tunnels were completed in 2007 and only minor works were continuing at the time of collapse.

Ground conditions along the metro were considered to be tricky, demanding a variety of stabilisation methods. The contractors used two of the Mixshield type of slurry boring machine.

Bilfinger Berger leads a three-firm joint venture which secured a $490 million (€390m) turnkey contract for over 3-km of line, at the south end, plus six stations, in November 2003.

A separate joint venture led by Hochtief Construction, Essen, handled the unaffected 700-m-long north section of line.