Why did Sands get arrested by the FBI?
Rebecca knew her apartment was bugged so she said she would kidnap Sands' son to draw him out. This caused Sands to show up with a gun to protect his son, at which point Boyle arrests him.
But what charges do they have to hold him on? As far as I know, Sands isn't wanted for anything by the FBI.
They cannot pin the surveillance cameras on him. Him responding to Rebecca's threat is circumstantial at best, and he can explain the gun away as being legal issue, or seeing the FBI with guns and fearing for the safety of his son. This is literally arresting him for thought-crime at this point.
They only suspect that he killed the evidence clerk or replaced the coat.
They can't prove he's behind the nzt purge, or the death of Rebecca's father.
Houstin cannot testify and no sane judge would accept testimony from a braindead criminal. Brian can't testify unless he wants to go to jail for being complicit in treason, murder, and obstruction of justice. He also can't testify for fear of his family's lives. Morra will deny all knowledge of any of his crimes.
When Sands was telling Rebecca that he killed her father, he spoke hypothetically, so that wasn't an admission of guilt.
So is the only crime they can pin on Sands is that he had a possibly illegal gun drawn in a residential area? If so, why did the FBI mobilize a strike force to take him down?