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The extreme solar and SEP event of 20 January 2005 is analyzed from twoperspectives. Firstly, we study features of the main phase of the flare, whenthe strongest emissions from microwaves up to 200 MeV gamma-rays were observed.Secondly, we relate our results to a long-standing controversy on the origin ofSEPs arriving at Earth, i.e., acceleration in flares, or shocks ahead of CMEs.All emissions from microwaves up to 2.22 MeV line gamma-rays during the mainflare phase originated within a compact structure located just above sunspotumbrae. A huge radio burst with a frequency maximum at 30 GHz was observed,indicating the presence of a large number of energetic electrons in strongmagnetic fields. Thus, protons and electrons responsible for flare emissionsduring its main phase were accelerated within the magnetic field of the activeregion. The leading, impulsive parts of the GLE, and highest-energy gamma-raysidentified with pi^0-decay emission, are similar and correspond in time. Theorigin of the pi^0-decay gamma-rays is argued to be the same as that of lowerenergy emissions. We estimate the sky-plane speed of the CME to be 2000-2600km/s, i.e., high, but of the same order as preceding non-GLE-related CMEs fromthe same active region. Hence, the flare itself rather than the CME appears todetermine the extreme nature of this event. We conclude that the acceleration,at least, to sub-relativistic energies, of electrons and protons, responsiblefor both the flare emissions and the leading spike of SEP/GLE by 07 UT, arelikely to have occurred simultaneously within the flare region. We do not ruleout a probable contribution from particles accelerated in the CME-driven shockfor the leading GLE spike, which seemed to dominate later on.

An Extreme Solar Event of 20 January 2005: Properties of the Flare and the Origin of Energetic Particles