Iodine-131 is used in cancer treatment so there may be a leak from pharmaceutical company. [0]

The similar case was observed in 2011 when the some I-131 isotopes leaked from the laboratory in Budapest. [1]

[0]: https://motherboard.vice.com/en_us/article/nobody-is-sure-wh...

[1]: http://journals.plos.org/plosone/article?id=10.1371/journal....