听力原文： Forensic experts yesterday began searching for clues in a bomb-shattered carriage lying 500 yards into one of the oldest and deepest Tube lines. The pace was slow and the approach delicate because the bodies of as many as 20 victims of the Piccadilly Line bomb near King's Cross were still trapped in the wreckage. After engineers declared the tunnel structurally safe, the aim was to remove the dead and start a fingertip examination. If the experts are lucky, they will find chemical traces, tiny scraps of bomb casing or detonation and timing devices. The carriage may be removed from the tunnel to allow better access but that could involve dismantling it, which could take many days. Similar searches started 100 yards into a tunnel near Liverpool Street station, where another of the bombs went off, and outside Edgware Road. Bodies were recovered and it was hoped that the two carriages could be removed. What are the forensic experts trying to do?