  An interval of exceptional solar activity was registered in early September
2017, late in the decay phase of solar cycle 24, involving the complex Active
Region 12673 as it rotated across the western hemisphere with respect to Earth.
A large number of eruptions occurred between 4-10 September, including four
associated with X-class flares. The X9.3 flare on 6 September and the X8.2
flare on 10 September are currently the two largest during cycle 24. Both were
accompanied by fast coronal mass ejections and gave rise to solar energetic
particle (SEP) events measured by near-Earth spacecraft. In particular, the
partially-occulted solar event on 10 September triggered a ground level
enhancement (GLE), the second GLE of cycle 24. A further, much less energetic
SEP event was recorded on 4 September. In this work we analyze observations by
the Advanced Composition Explorer (ACE) and the Geostationary Operational
Environmental Satellites (GOES), estimating the SEP event-integrated spectra
above 300 keV and carrying out a detailed study of the spectral shape temporal
evolution. Derived spectra are characterized by a low-energy break at few/tens
of MeV; the 10 September event spectrum, extending up to ~1 GeV, exhibits an
additional rollover at several hundred MeV. We discuss the spectral
interpretation in the scenario of shock acceleration and in terms of other
important external influences related to interplanetary transport and magnetic
connectivity, taking advantage of multi-point observations from the Solar
Terrestrial Relations Observatory (STEREO). Spectral results are also compared
with those obtained for the 17 May 2012 GLE event.
