  Light propagation and storing in a medium of atoms in the tripod
configuration driven by two control pulses are investigated theoretically in
terms of two polaritons and numerically. It is shown that a magnetic field
switched on at the pulse storage stage changes the phase relations between the
atomic coherences due to the stored pulse, which leads to an essential
modification of the released pulse. Quantitative relations concerning the
released pulse and the coherences are given. A general situation when the two
control fields are not proportional at the pulse release stage is also
examined. It is shown that in both cases a single dark state polariton is not
sufficient to account for the pulse evolution, which is connected with the fact
that a part of the signal remains in the medium after the release stage.
