Cruel and unusual punishment is banned by law in the USA. Except in one particular circumstance. If, by NOT ARRESTING SOMEONE, you cause them extreme mental anguish and even drive them to suicide, you cannot be punished.
Think it through carefully. By NOT arresting someone…
Now expand your thinking. What else do you accomplish by NOT arresting someone? You give them lots more opportunity to incriminate themselves and provide additional evidence. This is called giving them all the rope they need to well and truly hang themself.
There's more. You give them a great opportunity to incriminate their colleagues. They can't resist communicating with partners in crime. They cannot resist talking about the crimes and trying to organize a coverup. They ask their partnetr if the police asked any questions about X where X is a crime that they committed, but we had not yet realized it.
Don't worry, the Trump administration will reel them in when there is no more benefit for leaving them free, but until then, catch and release is the motto of this investigation.
It also helps with balancing and planning out the load on the courts. Because there are an awful lot of criminals out there that were involved. That's how ORGANIZED CRIME works and this was organized crime on a global scale. You will be ASTOUNDED when you learn how many foreigners were involved in this.