They had 20 tons of explosives and 71 chemicals mixed. The attack was planned in 1999. They had a rough estimate of the number of people that would be killed. Only a breach in operational security prevented them from achieving the objective. They will try again.
Remember the "cooked food secret" = timing + location + quality + quantity. They failed to keep the "secret" this time. We have to be 100% successful in detecting them to prevent another attack. They only have to be successful once.