From past news media reports, FBI only assisted with evidence analysis. Otherwise, they were kept at arm's length from the investigation.
JMO, prosecutors will have to reveal evidence against the defendants. It's the law. I can't envision the W's defense attorneys allowing them to avoid "discovery". If any of the defendants accept a plea bargain, it's possible some of the evidence will be kept hidden, but there's really no justification for keeping everything secret. It's not as if the evidence in these murder cases is a threat to national security or anything. The only exception might be identities of witnesses who came forward.
That said, this case is being tried in Ohio, and we've always joked that many Ohio pols think the US Constitution doesn't apply here.