Complex sentence: All of the precinct's occupants become targets once the gunmen's identities become apparent: they are a gang of crooked police officers led by Marcus Duvall (played by Gabriel Byrne), head of the Organized Crime and Racketeering Squad.
Split into simpler sentences: All of the precinct's occupants become targets once the gun men's identities become apparent. The rest of the ground in the precinct are told that they are a gang of crooked police officers led by Marcus Duvall (played by Gabriel Byrne), head of the Organized Crime and Racketeering Squad.