Anyone who wants to commit a serious crime with the help of others will physically destroy the evidence that leads to those helpers before committing the crime.

Anyone who wants to commit a serious crime with the help of others will physically destroy the evidence that leads to those helpers before committing the crime. Getting the phone data after the crime has been committed is not going to help preventing these crimes. Terrorists have been on the radar of secret agencies and police in countless cases. Unfortunately, if the police interviews a potential terrorist or mass killer for "behavioral irregularities" or whatever you call it, this will only radicalize the person more and prevent nothing. Or, at least, it is not a very effective intervention. If there is no crime, the police cannot do much. To be clear, the NSA could probably easily eavesdrop into all cell phone activity within the US and collect heuristic indicators, and it's very likely that they could predict most of these mass shootings quite well based on these indicators. The problem is that this would be unconstitutional and based entirely on thought crimes like "conspiring to commit a terrorist attack". The laws for this are in place and used against international terrorism, but they are highly problematic and shouldn't be extended to domestic terrorism. Social workers and maybe mental health support by people who do not have to report to the police probably work much better against radicalization and decrease the probability of an incident. I might be wrong but my guess is that in the US almost zero efforts are spent by social services to improve the social situation of right wing gun nuts and de-radicalize them. Again, no need for getting what's on their phone.