The behavior of runaway electrons of Frascati Tokamak Upgrade (FTU) discharges is investigated through the comparison of experimental synchrotron emission spectra and visible images with their synthetic counterparts. Synchrotron spectra are measured in an unprecedented wide wavelength range (450-4000 nm) while synchrotron images are collected by a visible CCD camera. The simulated spectra and images are calculated with the synthetic synchrotron radiation diagnostic SOFT (Synchrotron-detecting Orbit Following Toolkit) code. The aim of this work is to extend the study of runaway electrons dynamics in FTU also to post-disruption phases. The runaway number, radial profile, energy and pitch angle have been evaluated during their whole time evolution, from the start-up to the post-disruption phase, assuming a given runaway electrons (RE) distribution function. The runaway number is found to increase by two orders of magnitude after the disruption, while the energy and pitch angle maintain similar values before and after the disruption. The runaway electrons are mostly distributed in the core of the plasma. The inferred maximum RE energy and pitch angle are in agreement with the results of simulations based on a runaway electron test particle model.