During a magnetic substorm on October 16, 1973 a number of magnetospheric bursts of energetic particles were observed simultaneously by IMP-6 and IMP-7 in the magnetotail (at XSM>-32 Re, YSM>1.2 Re and ZSMSM?- 4.7 Re) while the two spacecraft were separated by only ~1 Re along the XsM axis (ΔXSM?1Re, ΔYSM=ΔZSm? Re). Detailed anisotropy measurements of 210 and 290 keV protons by the JHU/APL instruments on both spacecraft provide for the first time an indication of the location of the ''source'' of the energetic magnetospheric particles as well as evidence for its movement with speeds from ~30 to more than 80 km/sec in association with the intensification of the westward auroral electrojet during a magnetic bay at the station closest to the local time of the spacecraft (also local midnight). The size of the source was estimated to be ¿X?500 km. The observations indicate that in this case energetic particles are accelerated to >1.85 MeV in a moving and localized region in the geomagnetotail.