  Fiber bursts are a type of fine structure that is often superposed on type IV
radio continuum emission during solar flares. Although studied for many
decades, its physical exciter, emission mechanism, and association with the
flare energy release remain unclear, partly due to the lack of simultaneous
imaging observations. We report the first dynamic spectroscopic imaging
observations of decimetric fiber bursts, which occurred during the rise phase
of a long-duration eruptive flare on 2012 March 3, as obtained by the Karl G.
Jansky Very Large Array in 1--2 GHz. Our results show that the fiber sources
are located near and above one footpoint of the flare loops. The fiber source
and the background continuum source are found to be cospatial and share the
same morphology. It is likely that they are associated with nonthermal
electrons trapped in the converging magnetic fields near the footpoint, as
supported by a persistent coronal hard X-ray source present during the flare
rise phase. We analyze three groups of fiber bursts in detail with dynamic
imaging spectroscopy and obtain their mean frequency-dependent centroid
trajectories in projection. By using a barometric density model and magnetic
field based on a potential-field extrapolation, we further reconstruct the 3-D
source trajectories of fiber bursts, for comparison with expectations from the
whistler wave model and two MHD-based models. We conclude that the observed
fiber burst properties are consistent with an exciter moving at the propagation
velocity expected for whistler waves, or models that posit similar exciter
velocities.
