A period of extreme solar activity was observed in early September 2017, during the decaying phase of solar cycle 24. A large number of bright eruptions were registered, including a X9.3 flare on 6 September and a X8.2 flare on 10 September, the two strongest soft X-ray flares in almost 11 years. Both were associated with fast Coronal Mass Ejections (CMEs) and produced Solar Energetic Particle (SEP) events measured by several spacecraft. In particular, the second event was energetic enough to induce a Ground Level Enhancement (GLE) detected by the worldwide neutron monitor network, the second GLE of solar cycle 24. In this work we present a preliminary analysis of the September 2017 SEP events made with the CALorimetric Electron Telescope (CALET) onboard the International Space Station (ISS). We also investigate the relativistic electron precipitation (REP) events associated with the geomagnetic storms occurring in the same period. Data are compared with those of other space- and ground-based detectors.