BRST Cohomology and Nonlocal Conserved Charges

A relation is found between nonlocal conserved charges in string theory and certain ghost-number two states in the BRST cohomology. This provides a simple proof that the nonlocal conserved charges for the superstring in an AdS_5 x S^5 background are BRST-invariant in the pure spinor formalism and are kappa-symmetric in the Green-Schwarz formalism.