I am not going to bother to go find the actual numbers, because I've looked at them many times before. But to get it in the right ballpark . . . African American men between the ages of 18 to 45 account for four times as much violent crime in the U.S. as do age-matched (though not sure about poverty/education/class based) Caucasian American men. That disparity is probably at least as large or larger when comparing African American men to any other ethnic or racial group of men in the U.S.
That means absolutely nothing about how the supposed 'signs' of the spurious categorical system called "race" relate to things like innate aggressiveness, inherent propensity toward violence, or any other "racialist" model.
This is a cultural issue; the disparities observed are almost certainly derived almost entirely from cultural forces and secondarily from economic or class/status factors. The problem is that, in the world today, so pervaded with overzealous political correctness and irrational thinking guised as "sensitivity" that even saying there is any difference at all is liable to be labeled as being racist.
There are cultural forces, of myriad nature, that make young African men vulnerable to going down the path of criminal thug. Those sorts of cultural forces are not as predominant in other American sub-cultures. That IMHO, is the honest to goodness truth and to be afraid of the truth is not helpful. If we want to solve such social problems, we need to be open-minded and honest about what the upstream determinants might be.
So, with that said, I can entertain the prospect that something about Roma culture--being as they are, the analogously disenfranchised, minority diaspora of much of central, southern and eastern Europe--predisposes criminality. It certainly is not an unfamiliar stereotype though I do not have the sort of knowledge to know if there is any truth to the stereotype.
We really must not be afraid to speak about these things and to speak about them openly. Something about the context of Italian culture in the 1920s made fascism very appealing; you could say it predisposed a 'vulnerability' to the allures of totalitarianism. The same happened with Germany culture at about the same time and thereafter. Going back farther, something about French culture made it vulnerable to Napoleonic megalomania in his day, etc., etc.