Freedman-Townsend vertex from Hamiltonian BRST cohomology
  Consistent interactions among a set of two-form gauge fields in four dimensions are derived along a Hamiltonian cohomological procedure. It is shown that the deformation of the BRST charge and BRST-invariant Hamiltonian for the free model leads to the Freedman-Townsend interaction vertex. The resulting interaction deforms both the gauge transformations and reducibility relations, but not the algebra of gauge transformations. 
C. Bizdadea, E. M. Cioroianu, S. O. Saliu