  Solar flares and coronal mass ejections (CMEs) are closely coupled through
magnetic reconnection. CMEs are usually accelerated impulsively within the low
solar corona, synchronized with the impulsive flare energy release. We
investigate the dynamic evolution of a fast CME and its associated X2.8 flare
occurring on 2013 May 13. The CME experiences two distinct phases of enhanced
acceleration, an impulsive one with a peak value of ~5 km s$^{-2}$ followed by
an extended phase with accelerations up to 0.7 km s$^{-2}$. The two-phase CME
dynamics is associated with a two-episode flare energy release. While the first
episode is consistent with the "standard" eruption of a magnetic flux rope, the
second episode of flare energy release is initiated by the reconnection of a
large-scale loop in the aftermath of the eruption and produces stronger
nonthermal emission up to $\gamma$-rays. In addition, this long-duration flare
reveals clear signs of ongoing magnetic reconnection during the decay phase,
evidenced by extended HXR bursts with energies up to 100--300 keV and
intermittent downflows of reconnected loops for >4 hours. The observations
reveal that the two-step flare reconnection substantially contributes to the
two-phase CME acceleration, and the impulsive CME acceleration precedes the
most intense flare energy release. The implications of this non-standard
flare/CME observation are discussed.
