We discuss the first massive level of the open charged superstring in a flat four-dimensional spacetime. We show how the Rarita-Schwinger and Fierz-Pauli Lagrangians are retrieved for spin-3/2 and 2, respectively. In the absence of an electromagnetic background, We derive explicit equations of motion that describe the propagation of fields of charged spin-3/2 in flat space-time, which has been a long-standing problem. We briefly comment on use of amplitude computation to investigate different forms of the Weak Gravity Conjectures. This talk is based on [1-4].