Grenfell Tower fire: Why is blaze 'unprecedented'?

Fire safety experts say Grenfell Tower's exterior will be the main focus for investigators examining the rapid spread of the blaze, called "unprecedented" by London's fire commissioner.While no cause of the blaze has officially been determined, witnesses have described how flames lapped up and around the 24-storey tower block at alarming speed in the early hours.One told Sky News it took just 15 minutes for the fire to climb 17 floors.