They're not being any more secretive about this than any other case would be. The whole thing is being done before a grand jury at this point which has no power at all except to decide whether an actual court should be convened. Grand juries are brought together in order to investigate situations when it's not entirely clear whether a case exists or not.
You keep talking about confiscation of political materials. And while it's true they did take them, it was because the organization that they were investigating was politically motivated and since they were refusing to talk, that evidence became the only link they had to go on. And nobody has even suggested that he possession of such materials in itself was illegal. Nobody's suggested that they were going to face charges for the materials themselves... it's just evidence with which to continue the investigation for the people they are looking for.
The jailing happened because of the refusal to cooperate with the grand jury. You keep bringing up 5th amendment rights, but that only applies if you're at risk of incriminating yourself... and it doesn't even apply to grand juries anyway. Maybe that is all messed up, but it's nothing to be shocked or horrified over. We're talking about a rule that hasn't changed in well over 200 years. Nowhere does it say that you can refuse to cooperate with police investigations. If that were the way it worked, then the judicial system wouldn't have any teeth at all to investigate crimes.
You also all seem to be forgetting, or ignoring, that serious crimes did occur. Large amounts of property damage and assault on other people. This, all in the middle of an otherwise peaceful protest. By defending the violent ones you're giving a bad name to the more peaceful ones.
There's also the issue that the grand jury was convened before the crime was committed. That's not anything unusual. As said before, grand juries are meant to investigate. Given some kind of hint or warning that this might happen, it's not unusual that they would form some investigative body before the fact. Perhaps just to keep an eye on things, but also perhaps to look for evidence in the hopes they might prevent it from happening.
It's absolutely stupid that you treat this like some special case of abuse of power, since this is all just standard procedure and if it hadn't involved people with political motivations and ways to get their message out and stir up sympathy, then you'd never have said a word. If this had been a murder and these three people were obviously connected to the murderer somehow, but refusing to cooperate with the investigation to protect the murderer, would you be singing the same tune?
I am not a fan of the judicial system, I'm not a fan of the police, I'm not a fan of most of what these people claim to be fighting against, but the judicial system, the police, they're a necessary part of society. The methods they use can sometimes be considered questionable. But in this case, it seems to me they're doing exactly what they should be doing. And the 'anarchists', they're going about things the wrong way.