The existence and structure of steady gaseous detonation propagating in a packed bed of solid inert particles are analyzed in the one-dimensional approximation by taking into consideration frictional and heat losses between the gas and the particles. A new formulation of the governing equations is introduced that eliminates the well-known difficulties with numerical integration across the sonic singularity in the reactive Euler equations. The new algorithm allows us to determine that the detonation solutions as the loss factors are varied have a set-valued nature at low detonation velocities when the sonic constraint disappears from the solutions. These set-valued solutions correspond to a continuous spectrum of the eigenvalue problem that determines the velocity of the detonation.