Specifically, the preponderance of evidence points to an area around the Ural Mountains near the Russia-Kazakhstan border as the source of the emissions.*
The radioactive isotope detected at unusually high levels is Ruthenium-106. Ruthenium is a by-product of a radioactive material used in medicine, molybdenum-99, according to NPR. These days, radiation detection monitors are extremely sensitive, so even small leaks are eventually exposed. Read more...