The Justice Department has launched an investigation into a radical Northern Virginia prosecutor who allegedly discriminated against US citizens by offering preferential treatment “only to illegal alien” criminal defendants.   The DOJ notified Fairfax County Commonwealth’s Attorney Steve Descano on Wednesday that it is looking into potential civil rights violations related to his office’s plea bargaining, charging decisions and sentencing policies.  “Under my leadership, the Civil Rights Division will not allow local prosecutors to pick and choose winners based on their immigration status,” Assistant Attorney General Harmeet Dhillon said in a statement .  Attorney Steve Descano allegedly discriminated against US citizens by offering preferential treatment “only to illegal alien” criminal defendants.…