Perhaps the Dai li were the first metalbenders, there is some evidence why this is true, for example, the xin fu when transporting toph back to the beifong house, toph was hitting the metal of the box she was in the wagon, until Xin Fu said the following sentence: "You may think that you are the greatest earthbender in the world, but even you cannot bend metal". He says this as if someone has already managed to bend metal. That is, this factor that the Dai Li are the first metalbenders.