Russia' Federal Security Service (FSB) now says it has solved the case and that Ukraine is directly responsible.
Ukrainian officials had already denied any involvement in the explosion.
But in a statement carried by Russian media outlets, the FSB contends that the Ukrainian special services hired a contractor who arrived in Russia in mid-July with her young daughter.
It alleges she rented an apartment in the same building as Ms Dugin to learn about her life. The security agency also said the suspects used a Mini Cooper car which used three different sets of licence plates.
The suspect escaped to Estonia after the explosion, the FSB said.