As of March 2019, the license information on the FHTAGN homepage credits the following:
Open Game License v 1.0 Copyright 2000, Wizards of the Coast, Inc.
Legend, © 2011, Mongoose Publishing.
Unearthed Arcana, © 2004, Wizards of theCoast, Inc.
Delta Green: Agent’s Handbook, © 2016,Dennis Detwiller, Christopher Gunning,Shane Ivey, and Greg Stolze.
Delta Green: Need to Know, © 2016, Shane Ivey and Bret Kramer.
FHTAGN: Grundregelwerk, © 2017, Deutsche Lovecraft Gesellschaft e.V.; Julian Kluge, Volker Rattel.