  We analyze the spontaneous emission of a two-level atom interacting with a
special class of structured reservoirs of field modes with band gap edge
coinciding with the atomic transition frequency. The exact time evolution of
the population of the excited level is evaluated analytically through series of
Fox-$H$ functions. Over estimated long time scales, inverse power law
relaxations emerge, with powers decreasing continuously to 2 according to the
choice of the special reservoir. No trapping of the population of the excited
level emerges. The same results are recovered in presence of $N-1$ atoms, each
one in the ground state, described by the Dicke model. The power of the inverse
power law decay results to be independent of $N$. A critical number
$N_{\alpha}^{(\star)}$ is evaluated, such that, for $N \gg
N_{\alpha}^{(\star)}$, the inverse power law decay vanishes.
