Invisible Murder by Lene Kaaberbol & Agente Friis

This is the follow up to the 2011 debut The Boy in the Suitcase.  In this second novel, we once again meet Red Cross nurse Nina Borg.  This time she is off helping sick Hungarian Roma (gypsy) children.  Meanwhile, a law student searching for his Roma half-brother ends up at the same camp as Borg.  The student is also being tracked, by a counter-terrorism unit.  Soon it becomes apparent that the Roma children are getting sick because of a radioactive substance that is spreading and once again Borg finds herself in the middle of an explosive situation.  This novel has it all, a mystery, great action, and flawed but empathetic characters.