This work has been identified to be in the public domain and is free to use and reuse without restriction. You can copy, modify, distribute and perform the work, even for commercial purposes, all without asking permission. Attribution is recommended but not required.
Also see this detailed guide covering the copyright and ethical considerations of digital sampling of audio and video materials.
Why is this free to use?
Citizen DJ provides a subset of films from the National Screening Room that were identified to have been created by the U.S. government, thus are in the public domain.