The National Security Agency (NSA)
NSA was the original developer of SELinux. Researchers in NSA's National Information Assurance Research Laboratory (NIARL) designed and implemented flexible mandatory access controls in the major subsystems of the Linux kernel and implemented the new operating system components provided by the Flask architecture, namely the security server and the access vector cache.
The SELinux GitHub repositories
SELinux Project Wiki
The SELinux Notebook - The Foundations - 4th Edition
DigitalOcean: An Introduction to SELinux on CentOS 7