  We report observations of an X class flare on 2011 September 6 by the
instruments onboard the Solar Dynamics Observatory (SDO). The flare occurs in a
complex active region with multiple polarities. The Extreme-Ultraviolet (EUV)
Variability Experiment (EVE) observations in the warm coronal emission reveal
three enhancements, of which the third one corresponds to an EUV late phase.
The three enhancements have a one-to-one correspondence to the three stages in
flare evolution identified by the spatially-resolved Atmospheric Imaging
Assembly (AIA) observations, which are characterized by a flux rope eruption, a
moderate filament ejection, and the appearance of EUV late phase loops,
respectively. The EUV late phase loops are spatially and morphologically
distinct from the main flare loops. Multi-channel analysis suggests the
presence of a continuous but fragmented energy injection during the EUV late
phase resulting in the warm corona nature of the late phase loops. Based on
these observational facts, We propose a three-stage magnetic reconnection
scenario to explain the flare evolution. Reconnections in different stages
involve different magnetic fields but show a casual relationship between them.
The EUV late phase loops are mainly produced by the least energetic magnetic
reconnection in the last stage.
