»Scientist Spies« by Paul Broda has a detailed statement of what is currently in the public domain concerning Engelbert Broda’s spying activities. The National Archives, London, file provides links to the Archives' own website and access to the MI5 and other government files. Vassiliev et al. provides information from KGB files.
1. Paul Broda: »Scientist Spies. A Memoir of My Three Parents and the Atom Bomb«
2. Documents from the National Archive, London
3. Alexander Vassiliev et al.: »Spies. The Rise and Fall of the KGB in America«
4. Press articles
5. Other related items