Sampson was dismissed from his post on Wednesday after details emerged about his conduct while in charge at Bristol Academy.
"It took a difficult decision to terminate Mark's contract and it's important to understand the context," Glenn told Sky Sports News.
"Back in 2014, we were advised there were concerns about Mark's conduct at his time at Bristol Academy, from a safeguarding perspective. These inquiries were investigated by the FA, independently reviewed and concluded in 2015. At that point, provided Mark had some mentoring and training, he was deemed to pose no threat from a safeguarding perspective.
"The point of the investigation was to clear Mark from a safeguarding perspective as a participant in football. The issues raised that came to our attention last week was more about his conduct and how that pertained to him being an employee.
"The issues that concerned me on reading the report was the insufficient boundaries between that of a coach and player and which we judge was conduct that doesn't become an FA employee. Hence the decision to terminate his employment."