  The neutrinoless double beta (0nubb) decay experiment GERDA at the LNGS of
INFN has started physics data taking in November 2011. This paper presents an
analysis aimed at understanding and modeling the observed background energy
spectrum, which plays an essential role in searches for a rare signal like
0nubb decay. A very promising preliminary model has been obtained, with the
systematic uncertainties still under study. Important information can be
deduced from the model such as the expected background and its decomposition in
the signal region. According to the model the main background contributions
around Qbb come from Bi-214, Th-228, K-42, Co-60 and alpha emitting isotopes in
the Ra-226 decay chain, with a fraction depending on the assumed source
positions.
