Thanks for the book recommendation, I'll add it to my reading list.Markopolos compiled his evidence and arguments in a fairly extensive write-up and more or less published it (made it widely available anyway). I'm sure I have a copy of it somewhere. I will dig it up and post it here and you can judge the pre-arrest evidence for yourself.
This main point, IIRC, is that Madoff's claimed strategy was not capable of either handling the size he was running nor producing the preternaturally consistent returns he was claiming.
Sounds like the Markopolos data may have risen to the probable cause level. A claimed strategy not being capable of the size you're running or preternatural returns are most definitely not probable cause and I can't imagine the screaming that would happen if you started asking federal agents to determine what exactly is a natural vs preternatural return or a capable size for a given strategy. It would really be an unfair ask on the agents as well, even if you were somehow able to get someone with enough actual experience at a hedge fund to be able to approximate those decisions to go work for $120K/year at the FBI.