The four devices used to target innocent Tube and bus passengers during yesterday's lunch hour in London are all believed to have been nail bombs. Each of the explosive packages is thought to have been packed with nails, nuts and bolts in an attempt to create maximum casualties. Terrorism experts said the attacks were markedly different from those on July 7, which used conventional bombs to kill people with the impact of the blast. This time the terrorists were aiming to kill and cause horrific injuries on a wider scale with deadly shrapnel packed into the bombs. Their target was not early-morning commuters - but families and tourists travelling in the lunch hour. One security source said: "This was another attempt to bring mass murder to London. We have had an amazingly lucky escape."