Sharks attracted to steel

Cage diving was invented by a diver who survived an attack by a shark. Wanting to return to the sea he was inspired by the lions cage in Melbourne zoo.

At first it was thought the sharks were attracted to the people in the cage, but later it was observed that sharks also investigated empty cages. It turns out that they are attracted to the steel. I wonder if this is electrical effect, or if they are usually attracted to the iron in blood? Dissolving iron from the cage may seem similar to haemoglobin.