In one year 4,240 legal, again... legal immigrant checks were done by the FBI/ICE for risk factors/criminal history of legal immigrants re-entering the US that should have been denied entry.

 A 3 day timeframe is allotted for the FBI to provide a report, after this period it is up to Customs and Border Patrol if they want to allow access to the US.

  In at least 3,960 of those cases, the investigation into their background exceeded 3 days and access was granted to a "prohibited re-entry" at the discretion of the Customs and Border Patrol, requiring authorities to attempt to gain control of immigrants with a potentially dangerous criminal history.

 So this does not mean they will commit crimes, just that they have a history, and as such should be denied re-entry to the US.  

 As a legal immigrant they have the same rights as a US citizen when in the country.  

 Given the resistance to policy such as "stay in Mexico" should this trend continue considering the low number of denials compared to the large number of approvals and the probability of crime is low?  Or should the waiting period be extended?