https://ru.krymr.com/a/rossia-shpionazh-bolgaria/31173921.html
https://www.rosbalt.ru/piter/2021/03/29/1894418.html
https://www.thetimes.co.uk/article/edinburgh-professor-gave-names-to-fake-russia-spy-68n2n3bzj
https://www.inopressa.ru/article/26Mar2021/wsj/bulgaria.html
https://www.wsj.com/articles/how-an-alleged-russianspyring-used-cold-war-tactics-11616701381
https://www.inopressa.ru/article/23Mar2021/nzz/bulgarien.html