  We investigate the nature of the accelerated particles responsible for the
production of the gamma-ray emission observed from the middle-aged supernova
remnant (SNR) HB 21. The analysis of more than nine years of Fermi LAT data
leads to the observation of an extended emission positionally in agreement with
the SNR HB 21. The bulk of this gamma-ray emission is detected from the
remnant; photons up to $\sim$10\,GeV show clear evidence of curvature at the
lower energies. The remnant is characterized by an extension of $0^{\circ}.83$,
that is, 30\% smaller than claimed in previous studies. The increased
statistics allows us also to resolve a point-like source at the edge of the
remnant, in proximity to a molecular cloud of the Cyg OB7 complex. In the
southern part of the remnant, a hint of an additional gamma-ray excess in
correspondence to shocked molecular clouds is observed. The spectral energy
distribution of the SNR shows evidence of a break around 400 MeV, which can be
properly fitted within both the hadronic and leptonic scenario. The pion-decay
mechanism reproduces well the gamma rays, postulating a proton spectrum with a
slope $\sim 2.5$ and with a steepening around tens of GeV, which could be
explained by the energy-dependent escape of particles from the remnant. In the
leptonic scenario the electron spectrum within the SNR matches closely the
locally measured spectrum. This remarkable and novel result shows that SNR HB
21 could be a direct contributor to the population of Galactic electrons. In
the leptonic scenario, we find that the local electron spectrum with a break
around 2 GeV, closely evokes the best-fitting parental spectrum within this
SNR. If such a scenario is confirmed, this would indicate that the SNR might be
a source of Galactic background electrons.
