Outlining how a terrorist operation could work, Sieber said: "You could go to any building site, take a small bomb, and put it inside a wall. You could do that 12 or 15 times in one day. You could still explode it 10 years later."
Sieber lambasted the lax security that he says is prevalent on building sites, claiming that site managers "are not interested in any aspect of security". He added: "Nearly every building is open to this kind of attack. Security on sites should be just as good as security at airports, or even better."
Sieber's warning is based on his observations of terrorist groups such as the IRA and Germany's Red Army Faction. He said both groups have already exploded bombs at road works.
Last month a hand grenade and dynamite were discovered near the Olympic stadium in Athens, which will be used for the 2004 Olympics.