According to YNET news the Bulgarian police know the connection between Hezbollah and two of the perpetrators. The statement says "We have established that the two were members of the militant wing of Hezbollah," Tsvetanov said. "There is data showing the financing and connection between Hezbollah and the two suspects."
U.S. Democratic Senator Merkley airs support for Iran nuclear deal (Reuters)
from the article: Report: Bulgaria expected to blame Hezbollah, Iran for Burgas bombing