Out of college, the only job I had where I used my degree, I worked for a company that had a contract to do background checks for all 4 branches of the military, several government agencies, and over 100 private companies. One of the companies was Raytheon/L3. I was not a government employee but did government work and carried a government credential. The background check for L3 will consist of a record check in every city and county you've lived, worked, or attended school in the last 10 years (and is good for 5 years) and require a source interview with people that can verify those things. Seeing as how you've had the same job for 25 years and probably lived in the same house for over 10 years, it will probably go pretty fast. The source interviews will most likely be a boss or coworker and a neighbor. The records checks will be done by phone or email but can be done in person.
A Special Investigator will be contacting you for an appointment to clarify any timeline contradictions. One bit of advice is to be honest. They really don't care what you did, just when you did it. Time erases many automatic disqualifications, but contradictions between what you say and what your neighbors say (even if they think they are helping) throw up flags. Secrets are bad, and people can be coerced to keep them secrets.