My years of experience tell me the assassination attempt was not the work of an organized group. My experience tells me - and I've dealt with EEO/DEI policies for decades - the worst of civil servants end up at the top. Why do I think this? Because bureaucrats are so scared of losing their job, too many are woefully unqualified for their position, and expecting them to function at a high level of competence is unrealistic. The worst are promoted, because they're too hard to fire. Incompetent officials hire friends, family, yes-men, and sycophants, instead of those qualified.
Those entrusted with protecting Trump were feckless as a group. Not only were there not enough agents for security, those in charge had no idea how they left too many gaps exposed for what was taken as an opportunity by a disturbed young man. With a better detail, the news reports would have only reported how a young man was caught attempting to sneak into the event with a rifle.
If there is any single group to blame, then it is whoever promoted, and allowed, an unqualified individual the position of head of the Secret Service. That's a position that should be held by someone with years of law enforcement experience, some military experience, and a clear understanding politics are not acceptable to be practiced by those hired to protect important people. This administration failed, will fail again, and some people need to be fired.