Sampson was sacked on Wednesday after evidence of "inappropriate and unacceptable" behaviour from his time as Bristol Academy boss came to light.
"It's obviously a surprise the allegations that have come out [against Sampson] but I think it's for everyone else, the FA and the investigations team. The safeguarding investigation of 2014, Sampson being sent on an education course in 2015, Eniola Aluko's complaint in 2016 and Dame Tanni Grey Thompson's Duty of Care report published in April 2017 were all missed opportunities for the governing body to more closely examine the issues".
"There was a full investigation, a proper investigation, an assessment process and when the report concluded in March 2015 he was deemed not to be a safeguarding risk", Glenn said, per the Guardian.
A spokesman for Bristol Academy said: "In 2014 the club was made aware of the FA's investigation into allegations surrounding Mark Sampson".
It is only 12 months ago that Sam Allardyce was dismissed as manager of the men's team for unguarded comments to an undercover reporter, and the FA is still under the microscope for what it knew about predatory paedophiles working in football in the past.
He was present when Sampson was interviewed for the job but has said he was unaware of any of the allegations from Sampson's time at Bristol. On the wall, there was a list of the family and friends who were coming to watch us and I just happened to be next to Mark.
Collins said: " I think he (Glenn) has got hard questions to answer. 'Nigeria? Make sure they don't bring Ebola with them'. It might have been easier to take if it was about me alone. "I called my mum and she was absolutely disgusted".
Sampson was cleared of wrongdoing in both instances by the FA, though the Professional Footballers' Association, the players' union in England, has called the internal review "a sham which was not created to establish the truth but meant to protect Mark Sampson".
"Speaking on Sky News, he said: "(Sampson) should never have been appointed in the first place and action should have been taken a long time ago. According to multiple reports, the FA did not talk to other England players who were present when Sampson allegedly made the racist statements.
Glenn and fellow under-fire chairman Greg Clarke will have to answer questions at a parliamentary inquiry next month.
He added on Sky News: "Do we need nearly a new body that sits outside of the FA that can look at serious issues like this that require proper and full investigation?"
Collins, chairman of the Culture, Media and Sport Select Committee, last night said: "If it transpires that FA executives had warnings and advice that they didn't follow up, then their positions become untenable".