Romania does have an extradition treaty with the US.  Cyber criminals from Romania have been extradited to the US before so this will not be something unheard of.  I would imagine they would notify the FBI of cyber fraud, provide evidence, and then let the chips fall as they may....

http://www.fbi.gov/newyork/press-releases/2013/three-members-of-international-cyber-fraud-ring-extradited-from-romania-to-the-united-states


Edit to add: this is of course assuming that the "they" you are referring to are US nationals.