Anastasiia Berezovska, 39, was found dead with gunshot wounds to the head during a crime-scene reconstruction near Kyiv, after Interpol had named her the main suspect in the June 29 Monaco bombing.
An officer in Ukraine's military intelligence agency confessed to killing her with help from a former law enforcement officer, telling investigators he acted on his own and without informing superiors.
SBU investigators said they zeroed in on the two men after tracing repeated cryptocurrency transfers and bank payments to Berezovska; both are now detained on suspicion of premeditated murder.
The Monaco blast targeted businessman Vadym Yermolaiev and his family, injuring three people including a child, and Monaco's Prince Albert II called it an "odious act."
Ukraine said it has shared its findings with Monaco and is still trying to identify who ordered and organized the attempted killing there.