The dirty bomb thing isn't anything new, and definitely precedes this. Both google and bing have blocked all RT.com results here in Germany, duck.com luckily still lets me search for it.

2009: https://www.rt.com/news/the-price-of-mass-destruction/

>Ukraine’s security service has arrested a group suspected of trying to sell a container of radioactive material. Officials say there was enough of the substance to create a dirty bomb, which could have posed a serious threat to the entire region.

2014: https://www.rt.com/news/ukraine-nuclear-arsenal-threat-314/

> While the uranium Ukraine gets from Russia for its reactors is low-enriched, reactor waste is quite enough to make a so-called dirty bomb. The country possesses manpower and know-how to do that as well as delivery vehicles for nuclear payload, which don’t have to be too sophisticated.

2015: https://www.rt.com/op-ed/311869-uranium-ukraine-nuclear-poro...

> According to Ukraine's security forces, they have seized nuclear material from a criminal gang that was trying to sell it. The substance is said to be Uranium-238, which can be used to create a so-called 'dirty bomb'

edit: Clarification for the many downvoting, this is in response to OP specifically saying "This makes the letter seem more plausible, if it really preceeds the news".