Pretty sure cops can and have been corrupt...And that is the way it should be. Basically, you need someone who is unimpeachable. We see cases of insiders at banks and in finance using their position to steal or to conspire with those who steal. With the popularity of "crime and punishment" shows on TV, and the stories in real life, no one is going to take a chance with a security person unless they are 110% competent and clean as the driven snow.
I was talking with someone with 100s of thousands in gold and precious metals at a show and he gave a scenario how organized crime would pull off a robbery at a show: hold a lady hostage and then use that to get all the dealers to capitulate and hand over their guns and valuables. I'm surprised that a major heist has not occurred already with all the $$$ so easily fenced with the Russian or Mexican mafias.