The Department of Homeland Security clarified that the data to

Avalon - The Department of Homeland Security clarified that the data to which Trump appeared to refer lists individuals who entered the country over the past 40 years—including during his own term—committed crimes in the U.S. rather than their country of origin, and either are currently incarcerated or have served their sentences but can’t be deported because their country of origin won’t accept them. Such individuals are monitored.