  On the basis of the Wigner unitary representations of the covering group
ISL(2,C) of the Poincar\'{e} group, we obtain spin-tensor wave functions of
free massive particles with arbitrary spin. The wave functions automatically
satisfy the Dirac-Pauli-Fierz equations. In the framework of the two-spinor
formalism we construct spin-vectors of polarizations and obtain conditions that
fix the corresponding relativistic spin projection operators (Behrends-Fronsdal
projection operators). With the help of these conditions we find explicit
expressions for relativistic spin projection operators for integer spins
(Behrends-Fronsdal projection operators) and then find relativistic spin
projection operators for half integer spins. These projection operators
determine the nominators in the propagators of fields of relativistic
particles. We deduce generalizations of the Behrends-Fronsdal projection
operators for arbitrary space-time dimensions D>2.
