A man convicted for plotting mass murder at the University of Helsinki in 2014, also planned to rob a Helsinki gun store, according to prosecutors.
The man allegedly sought accomplices for the crime on the Tor network, which is also known as the Dark Web and used for anonymous communications.
However his scheme was outed when an undercover police officer responded to his request for a partner in crime.
The suspect is currently facing charges of aggravated plotting to endanger life or health, which were filed last December over suspicions that he was behind another plot to commit mass violence.
He has also been charged with drug offences as well as being in possession of objects or substances designed to inflict harm.
The man and a female accomplice had previously served a three-year prison sentence for his earlier conviction on charges of plotting mass killings at the university. The woman later had her sentenced overturned on health grounds.