  We investigate the saturation of particle acceleration in relativistic
reconnection using two-dimensional particle-in-cell simulations at various
magnetizations \sigma. We find that the particle energy spectrum produced in
reconnection quickly saturates as a hard power law that cuts off at
\gamma~4\sigma, confirming previous work. Using particle tracing, we find that
particle acceleration by the reconnection electric field in X-points determines
the shape of the particle energy spectrum. By analyzing the current sheet
structure, we show that physical cause of saturation is the spontaneous
formation of secondary magnetic islands that can disrupt particle acceleration.
By comparing the size of acceleration regions to the typical distance between
disruptive islands, we show that the maximum Lorentz factor produced in
reconnection is \gamma ~ 5 \sigma, which is very close to what we find in our
particle energy spectra. We also show that the dynamic range in Lorentz factor
of the power law spectrum in reconnection is < 40. The hardness of the power
law combined with its narrow dynamic range implies that relativistic
reconnection is capable of producing the hard narrowband flares observed in the
Crab Nebula but has difficulty producing the softer broadband prompt GRB
emission.
