  Neutrinoless double beta decay is a process that violates lepton number
conservation. It is predicted to occur in extensions of the Standard Model of
particle physics. This Letter reports the results from Phase I of the GERmanium
Detector Array (GERDA) experiment at the Gran Sasso Laboratory (Italy)
searching for neutrinoless double beta decay of the isotope 76Ge. Data
considered in the present analysis have been collected between November 2011
and May 2013 with a total exposure of 21.6 kgyr. A blind analysis is performed.
The background index is about 1.10^{-2} cts/(keV kg yr) after pulse shape
discrimination. No signal is observed and a lower limit is derived for the
half-life of neutrinoless double beta decay of 76Ge, T_1/2 > 2.1 10^{25} yr
(90% C.L.). The combination with the results from the previous experiments with
76Ge yields T_1/2 > 3.0 10^{25} yr (90% C.L.).
