Yes, Americans should report extremist threats if they are aware of an upcoming attack. This helps ensure the safety of themselves and their fellow citizens. However, there should be some sort of guidelines for what constitutes an extremist threat so as to avoid citizens clouding the justice system with false accusations based on misinformation or bigotry.
It should be common sense that one should see something and then report it. There are so many people who ignore what they see and admit later that they knew what was going to happen. For most of us it is simply unthinkable. There should be repercussions for such blatant disregard for human life. Don't assume if someone tells you they are going to slaughter innocent people that they are not serious or just kidding. It is better to err on the side of caution.
Americans should be required to report extremist threats. Prior knowledge of extremist threats demonstrates prior knowledge of serious crime. If those who know beforehand of an extremist threat do not report it, they should be charged as an accessory to the crime. Law enforcement should be given an opportunity to investigate threats. The only way they can effectively do that is if those with prior knowledge report extremist threats.
Extremist threats are a national security issue at both the political and personal levels. This means that all Americans should do their part to report these threats so that the proper authorities can address them and continue to keep us safe. We should make this a national priority by requiring our citizens to do so.